How to Measure for a New Toilet: A Step-by-Step Buying Guide

Make Sure Your New Toilet Fits Before You Order

Buying a new toilet isn't quite as simple as finding one you like the look of and clicking Add to Basket.

Two toilets can look almost identical online but have completely different:

• Projections
• Heights
• Widths
• Waste connections
• Cistern dimensions
• Pipework requirements

And discovering that your beautiful new WC doesn't fit after the old toilet has been removed isn't anyone's idea of a relaxing bathroom renovation.

Fortunately, a few simple measurements can help you narrow down the right options before you buy.

In this Allbits guide, we'll show you how to measure for a new toilet, what dimensions matter and what you need to check when choosing between close-coupled, back-to-wall and wall-hung WCs.

Before You Start: What Will You Need?

Nothing particularly technical.

Grab:

• Tape measure
• Pencil
• Paper or phone for notes
• Masking tape
• Your existing toilet measurements if you're replacing one

If possible, measure everything in millimetres.

Bathroom product specifications are generally given in millimetres, so this makes comparing your measurements with the product information much easier.

💡 Big Al's Expert Tip

Measure everything twice and write it down. Do not trust yourself to remember whether the gap was 600mm, 660mm or "about that big." 😂

The 7 Measurements and Checks That Matter

Before choosing a toilet, we want to establish:

Let's tackle them one at a time.

1. Measure the Toilet Projection

Projection means how far the toilet extends from the rear installation point into the bathroom.

For a conventional close-coupled WC, measure from:

Rear of toilet/cistern → furthest front edge of pan.

This is one of the most important measurements in:

• Small bathrooms
• Cloakrooms
• En-suites
• Narrow bathrooms

A shorter projection can give you extra space in front of the WC.

For example, the current Roca The Gap Square Rimless Closed-Back Close-Coupled Toilet at Allbits has a compact 600mm projection, with overall dimensions of 365mm wide × 600mm deep × 790mm high.

View Roca The Gap Square at Allbits

For much more on this measurement, read:

Toilet Projection Guide: How to Choose the Right Size Toilet

2. Measure the Available Width

Next, measure the width of the space available for the toilet.

Don't just ask:

"Will the ceramic physically fit?"

You also need comfortable space around it.

Consider nearby:

• Walls
• Vanity units
• Baths
• Shower enclosures
• Radiators
• Pipe boxing
• Furniture

Measure at several points if the room isn't perfectly square.

Also check the toilet's widest measurement, which may be the cistern rather than the pan.

For comparison, your current Allbits MyHome Fully Back-to-Wall Close-Coupled WC is 365mm wide and 600mm deep.

3. Check the Toilet Height

There are actually two heights worth thinking about.

Overall Height

This is the measurement from the floor to the highest point of the toilet, usually the top of a close-coupled cistern.

This matters if the toilet is going beneath:

• Window sill
• Shelf
• Boiler boxing
• Cupboard
• Sloping ceiling

Pan Height

This is the height from the floor to the top of the ceramic pan before, usually, accounting for the seat.

It determines the basic sitting height.

This becomes especially important if you're considering a comfort-height toilet.

For example, the Roca The Gap Square Comfort Height Back-to-Wall Pan currently listed by Allbits has a 440mm pan height and 610mm projection.

View Roca The Gap Square Comfort Height at Allbits

Read:

Comfort Height Toilets Explained: Are They Right for You?

4. Check the Waste Outlet and Soil Pipe

Now we get to the bit that isn't particularly glamorous but is very important.

Your toilet needs to connect to the property's soil pipe.

Before buying a replacement WC, identify:

• Where the existing waste exits
• Whether it goes through the wall or floor
• Approximate centre position
• Available space for the pan connector
• Whether existing boxing restricts access

Don't assume a new toilet will connect exactly like your existing one.

Some modern toilets use a dual outlet arrangement to provide greater installation flexibility. The Roca The Gap Square close-coupled WC, for example, is currently specified by Allbits with a dual outlet.

💡 Big Al's Expert Tip

If you're uncertain about the waste connection, take photographs and measurements before ordering and speak to your installer or the Allbits team. A toilet being 20mm shorter isn't much help if connecting it to the existing soil pipe becomes a plumbing expedition.

5. Check the Water-Supply Position

This is particularly important with modern closed-back toilets.

An open-back WC leaves more access around the rear of the pan.

For example, Allbits describes the Roca Andes Rimless Close-Coupled WC as an open-back design that allows straightforward access to pipework. 

A closed-back toilet conceals more of the plumbing.

That looks lovely.

But your installer needs to ensure the existing:

• Water supply
• Isolation valve
• Waste connection

can be accommodated.

This is why replacing an old open-back toilet with a sleek closed-back WC deserves a little planning.

Open Back vs Closed Back When Replacing a Toilet

This distinction is worth understanding.

Open-Back Toilet

The rear sides of the pan are open or cut away.

Advantages

• More access to plumbing
• Useful for existing pipework
• Can simplify some replacement installations

Closed-Back Toilet

The ceramic extends towards the wall and conceals more of the plumbing.

Advantages

• Cleaner appearance
• Less visible pipework
• Fewer awkward external areas to clean

But don't choose closed back purely because it looks tidier.

Check your plumbing first.

6. Measure the Space in Front of the Toilet

A toilet fitting into the room isn't the same as a toilet being comfortable to use.

Measure from the proposed front edge of the pan to anything opposite it, such as:

• Wall
• Bath
• Shower
• Vanity unit
• Door
• Furniture

You need enough room to approach, sit and stand comfortably.

This is where our favourite cheap bathroom-planning tool comes back out:

Masking Tape!

Find the dimensions of the toilet you're considering.

Then mark its approximate footprint on the floor.

Suddenly a specification such as:

365 × 600mm

becomes something you can actually see.

Walk around it.

Open the door.

Stand in front of it.

Pretend the toilet is there.

Yes, you'll look slightly peculiar.

But you'll look considerably more peculiar trying to squeeze sideways past a badly chosen WC for the next 15 years. 🤣

7. Check the Bathroom Door

Do not forget this one.

Especially in a cloakroom.

Check:

• Which way the door opens
• Whether it swings over the toilet footprint
• Whether the door handle clashes with anything
• Whether you can comfortably enter and leave

A compact toilet can sometimes make a huge difference.

Our previous guide covers this in detail:

Best Toilets for Small Bathrooms

Measuring a Close-Coupled Toilet

A close-coupled WC is usually the most straightforward arrangement to measure.

You need:

Overall Projection

Rear of cistern to front of pan.

Overall Width

Check the widest point.

Overall Height

Floor to top of cistern.

Pan Height

If sitting height matters.

Waste Position

Check compatibility with existing soil connection.

Water Supply

Check where the cistern will be fed.

If you're replacing an existing close-coupled WC with another similar model, taking detailed measurements of the existing installation gives you a useful starting point.

Measuring for a Closed-Back Close-Coupled Toilet

This needs a little more care.

Because the ceramic extends towards the wall, there may be less room behind the pan for existing:

• Pipes
• Valves
• Connectors

The Roca The Gap Square Rimless is a good example of a modern closed-back WC. It gives you a tidy, streamlined installation and compact 600mm projection, but its product information specifically notes the enclosed rear design.

So if you're replacing an older open-back toilet, don't assume the existing plumbing will automatically disappear neatly behind it.

Ask your installer to check.

Measuring for a Back-to-Wall Toilet

A true back-to-wall toilet has a floor-standing pan but no visible cistern sitting above it.

The cistern is concealed:

• Inside furniture
• Behind a false wall
• Inside a service void

That means measuring the pan alone doesn't tell you how much room the finished installation will use.

You need to calculate:

Concealed cistern/furniture depth + pan projection from finished surface

And remember to allow appropriate servicing access to the concealed cistern.

Read:

Back-to-Wall Toilets Explained: Everything You Need to Know

Measuring for a Wall-Hung Toilet

Wall hung is where people can get caught out.

The ceramic pan may have a beautifully compact projection.

For example, the current Roca Inspira Square Rimless Wall-Hung Pan at Allbits measures 370mm wide × 560mm deep × 440mm high.

View Roca Inspira Square Wall Hung at Allbits

But that doesn't mean the complete installation occupies only 560mm from your original wall.

Behind the finished surface you may also need:

• WC frame
• Concealed cistern
• Pipework
• Wall construction
• Board
• Tiles

Therefore calculate:

Original wall → finished wall face → front of pan.

💡 Big Al's Expert Tip

With wall-hung toilets, compare the complete finished installation, not just the ceramic pan measurement.

Read:

Wall-Hung vs Close-Coupled Toilets: Which Is Best?

What If You're Completely Changing the Bathroom Layout?

If the toilet is moving to a different part of the room, measuring the existing WC won't be enough.

You'll need to consider:

• New waste route
• Soil-pipe connection
• Water supply
• Floor construction
• Wall construction
• Ventilation requirements where relevant
• Clearances
• Other bathroom products

Moving a toilet can sometimes involve considerably more plumbing work than simply replacing one in roughly the same position.

Discuss the proposed layout with a suitable installer before ordering the sanitaryware.

Measure the Other Bathroom Products Too

Your toilet does not live in splendid isolation.

Measure the:

• Basin
• Vanity
• Bath
• Shower
• Towel rail
• Furniture

Then consider how they interact.

For example:

Toilet + Vanity

Will drawers and doors open?

Toilet + Shower

Will the shower door clear the WC?

Toilet + Bath

Is there comfortable movement between them?

Toilet + Towel Rail

Will the toilet position interfere with the radiator or make it awkward to use?

Toilet + Door

Can you actually get into the room?

Bathroom planning is really about the whole room, not individual products.

Replacement Toilet Measuring Example

Let's imagine your existing toilet measures:

670mm projection.

You find a new model with:

600mm projection.

On paper, you've saved:

70mm.

Brilliant.

But before ordering, also check:

• Width
• Height
• Waste outlet
• Water inlet
• Pan shape
• Pipe clearance

If those all work, you've potentially gained a useful 70mm of floor space without changing the basic bathroom layout.

This is why replacing an older WC with a modern compact model can make a small bathroom feel noticeably less crowded.

Should the New Toilet Be the Same Size as the Old One?

No.

It just needs to work with the available space and plumbing.

You might deliberately choose a:

• Shorter toilet
• Taller comfort-height toilet
• Narrower WC
• Closed-back toilet
• Rimless toilet

The existing toilet gives you a useful reference point.

It isn't a template you're forced to copy.

What Measurements Are Shown on an Allbits Product Page?

This varies by product, but you'll commonly find dimensions such as:

• Width
• Projection/depth
• Overall height
• Pan height

For example, current Allbits listings show the MyHome Fully Back-to-Wall Close-Coupled WC at 365 × 600 × 780mm, while the Evora Rimless Closed-Sided WC is listed at 615mm projection with a 430mm pan height.

Always check the specific product page and technical information before ordering rather than assuming another WC from the same range has identical measurements.

Don't Forget What's Included

This isn't technically a measurement...

...but while you've got the specification open, CHECK IT. 😂

Depending on the toilet, you may need:

• Pan
• Cistern
• Seat
• Fixings
• Concealed cistern
• WC frame
• Flush plate
• Pan connector

Packages vary.

For example, the current Roca The Gap Square close-coupled listing includes the pan, compact cistern, soft-close seat and fixing kit, whereas a wall-hung arrangement has different installation requirements.

Our previous guide explains the seat side:

Soft-Close Toilet Seats Explained: Are They Worth It?

Big Al's Measure-Before-You-Buy Method

If I wanted to make this really simple for an Allbits customer, I'd do it in this order:

Step 1

Measure the room.

Step 2

Measure the existing toilet.

Step 3

Photograph the existing waste and water connections where visible.

Step 4

Choose your preferred WC type:

Close coupled / back to wall / wall hung.

Step 5

Check the new toilet's:

Width + projection + height.

Step 6

Check the waste and water connection requirements.

Step 7

Mark the proposed footprint with masking tape.

Step 8

Check:

Door + basin + shower + bath + furniture clearance.

Step 9

Confirm what's included.

Step 10

If you're uncertain about installation compatibility:

Ask before ordering.

Ten minutes spent measuring can save an awful lot of:

"Oh..." when the plumber opens the box. 😂

Your Complete Toilet Measuring Checklist

☐ Measure room width

☐ Measure room depth

☐ Measure ceiling height if relevant

☐ Measure existing toilet projection

☐ Measure available maximum projection

☐ Measure available toilet width

☐ Check overall toilet height

☐ Check pan height

☐ Identify soil-pipe position

☐ Check waste outlet direction

☐ Check water-supply position

☐ Locate isolation valve

☐ Check whether existing pipework suits an open- or closed-back pan

☐ Check bathroom-door swing

☐ Check basin clearance

☐ Check vanity-door/drawer clearance

☐ Check shower-screen or shower-door clearance

☐ Check bath clearance

☐ Check radiator/towel-rail position

☐ For back-to-wall WCs, include furniture/boxing depth

☐ For wall-hung WCs, include frame and finished-wall depth

☐ Mark proposed toilet footprint with masking tape

☐ Check whether cistern is included

☐ Check whether seat is included

☐ Check whether seat is soft close

☐ Check whether fixings are included

☐ Check product technical information

☐ Confirm installation requirements with your installer

☐ Measure everything again

Common Toilet Measuring Mistakes

Measuring Only the Pan

Measure the complete finished installation.

Assuming All Toilets Are Roughly the Same Size

They absolutely aren't.

Forgetting the Soil Pipe

Our old friend has made it to the finale. 😂

Forgetting the Water Supply

Particularly troublesome with some closed-back designs.

Measuring to an Unfinished Wall

Tiles, furniture and boxing can alter finished dimensions.

Forgetting Wall-Hung Frame Depth

The frame hasn't vanished.

It's merely hiding.

Forgetting Back-to-Wall Furniture Depth

Same problem, different disguise.

Ignoring the Door Swing

A toilet that blocks the door is technically installed.

It's just not particularly useful.

Choosing on Projection Alone

Width and height matter too.

Not Checking What's Included

A toilet photograph may show components that aren't necessarily part of the package.

Measuring Once

You know what I'm going to say.

Measure twice. Buy once.

Frequently Asked Questions

How Do I Measure for a Replacement Toilet?

Measure the available width, projection and height, then check the existing waste outlet, soil-pipe arrangement and water supply.

Compare those measurements with the new toilet's technical specification.

What Does Toilet Projection Mean?

Projection is how far the toilet extends into the bathroom from its rear installation point.

Read our complete Toilet Projection Guide for more information.

What Is the Most Important Toilet Measurement?

There isn't just one.

Projection is particularly important for room space, but width, height and plumbing compatibility all matter.

Can I Replace Any Toilet with Any Other Toilet?

Not necessarily.

Different toilets have different dimensions, outlet arrangements and plumbing requirements.

How Do I Know Whether I Need an Open-Back Toilet?

An open-back pan can be useful where access to existing pipework is required.

Closed-back designs conceal more plumbing but may need greater planning around pipe positions.

How Do I Measure for a Wall-Hung Toilet?

Consider both the pan dimensions and the complete frame/concealed-cistern installation, including the finished wall depth.

How Do I Measure for a Back-to-Wall Toilet?

Include the depth of the furniture or concealed-cistern wall plus the pan projection from that finished surface.

What Is a Good Toilet Projection for a Small Bathroom?

There's no universal ideal, but models around 600mm and below are worth investigating where space is tight. The actual room layout and plumbing still determine what works.

Can I Replace a Standard Toilet with a Comfort-Height Toilet?

Potentially, yes, provided the new WC dimensions and plumbing arrangement suit the installation.

Should I Measure the Toilet With the Seat?

For overall planning, use the manufacturer's stated product dimensions and check how those dimensions are defined. For sitting comfort, check the pan height and seat specification separately.

What If I'm Not Sure My New Toilet Will Fit?

Don't guess.

Take measurements and photographs and discuss them with your installer.

Final Thoughts

The golden rule of toilet buying:

Measure first. Choose second.

A toilet needs to do more than look good in a photograph.

It needs to:

• Fit the available space
• Connect to the plumbing
• Leave comfortable room around it
• Work with the bathroom layout
• Suit the people using it

Start with the room.

Measure the existing installation.

Check the plumbing.

Compare the technical specification.

Mark the footprint on the floor.

And if anything doesn't make sense, ask before ordering.

Because the best toilet isn't necessarily the most expensive, the smallest or the fanciest.

It's the one that fits your bathroom properly.

Browse the Allbits toilet range:

Shop Toilets at Allbits

Related Allbits Toilet Buying Guides

• The Ultimate Guide to Buying a Toilet
• Rimless Toilets Explained: Are They Worth It?
• Wall-Hung vs Close-Coupled Toilets: Which Is Best?
• Back-to-Wall Toilets Explained
• Comfort Height Toilets Explained
• Toilet Projection Guide
• Best Toilets for Small Bathrooms
• Traditional vs Modern Toilets
• Soft-Close Toilet Seats Explained

• What is a Bidet? Complete Bidet Buying Guide