Accurate measuring is the first step when choosing foam sheets, replacement cushions or mattress toppers. The right product will not give a reliable result if the finished piece is too wide, too short, too deep or based on a compressed old insert. Measure the space, cover or item the new foam must fit, record the dimensions clearly and compare them with the exact options shown on the product page.
The measuring method changes with the project. A rectangular seat cushion can usually be measured from its cover. A window seat may need several measurements because the walls are not square. A protective case insert must account for both the case and the item inside it. A mattress or topper should be checked against exact centimetre dimensions rather than the size name alone.
This guide explains how to measure each type of project and how to avoid the mistakes that most often lead to a poor fit.
Start with the finished space
Measure the place where the new foam will be used, not only the foam that is being replaced.
Old foam can become compressed, rounded or permanently indented. If you copy those changed dimensions, the replacement may also be too small. A seat cushion that has spent years under load may no longer fill its cover. A case insert may have been trimmed after purchase. A mattress may have softened at the edges and no longer represent its original width or depth.
Use the most stable reference available.
For a removable cushion, this is usually the empty cover measured from seam to seam. For a fixed bench, it is the usable platform or recess. For packaging, it is the internal case size and the item that needs protection. For a topper, it is the mattress surface. For a replacement mattress, it is the bed frame or base together with the listed mattress dimensions.
Before recording any number, decide what the finished foam must fit. This prevents you from mixing measurements from different references.
Record width, length and thickness separately
Every rectangular foam piece needs three dimensions.
Width is the measurement from side to side.
Length is the measurement from front to back or from head to foot, depending on the product.
Thickness is the depth of the foam from its top surface to its bottom surface.
Write the dimensions in the same order every time. A simple record such as width, length, thickness reduces the chance of swapping two values when you place the order.
Include the unit beside every number. Do not rely on memory to tell you whether a figure was recorded in millimetres, centimetres or inches.
For small and precise projects, millimetres are useful because they reduce early rounding. You can convert the final measurement when comparing it with a product option. One centimetre equals ten millimetres. One inch equals 25.4 millimetres.
Do not mix units within one set of measurements unless the product options require it. A note such as 760 mm wide, 100 cm long and 2 inches thick is harder to check than one consistent set.
Use suitable measuring tools
A flexible tape measure is useful for large furniture, mattresses and case interiors. A rigid rule or steel tape is often easier for smaller foam pieces and straight edges.
Place the item on a flat surface where possible. Keep the tape straight and level. A tape that bends around a cushion edge can produce a longer reading than the actual finished width.
Measure each important dimension more than once. For a wide bench or uneven opening, take readings at several points rather than assuming both ends are identical.
Record the smallest and largest measurements if they differ. Do not silently average them. The difference may show that the space is tapered, bowed or out of square and needs a template or further advice.
How to measure a rectangular foam sheet
For a simple rectangle, measure the required finished width and length across the flat surface. Then measure the required thickness separately.
Check the direction in which the piece will sit. A rectangle measuring 80 cm by 50 cm will physically rotate within some sheets, but orientation can matter when it must line up with a cover, a shaped edge or several related pieces.
Compare the finished dimensions with the available sheet size before ordering. Foam sheet dimensions vary by product. Many upholstery grades are available in 76 cm by 100 cm, 230 cm by 100 cm and 230 cm by 200 cm sheets. Other products use different sizes, including 100 cm by 100 cm, 200 cm by 100 cm, 150 cm by 100 cm or 100 cm by 120 cm.
Do not assume that every foam grade uses the same stock dimensions. Open the exact product card and check its size selector.
If you plan to cut several pieces from one sheet, draw a cutting plan first. Each rectangle must fit within the chosen sheet width and length. Rotating one or more pieces may use the material more efficiently, but check that every required piece still fits.
The foam sheet thickness guide explains how depth affects cushioning, support and clearance after the width and length have been confirmed.
How to measure replacement seat cushions
A removable seat cushion should usually be measured from the empty cover rather than the old foam.
Remove the insert carefully and place the cover on a flat surface. Smooth the fabric without stretching it. Measure from seam to seam across the width and length. Then measure the side panel or border to understand the intended thickness.
Take several readings if the cover is not perfectly rectangular. The front may be wider than the back. One side may be longer. Rounded corners may begin at different points.
Do not automatically add extra width, length or thickness. Some covers are designed for a snug fill, but the correct allowance depends on fabric stretch, seam construction, wrapping materials and the way the cushion is assembled. An arbitrary addition can make the cover difficult to close or distort its shape.
If the cushion includes polyester wrap, wadding or another outer layer, record this separately from the foam core. The cover measurement represents the complete insert, not necessarily the bare foam dimensions.
The upholstery foam guide explains how seat bases, back cushions, benches and headboards place different demands on foam grade and thickness.
How to measure back cushions
Back cushions often have softer shapes than seat cushions. They may taper, curve or include a rounded crown.
Start with the empty cover where possible. Measure the widest point, the full height and the intended depth. Check whether the top and bottom widths differ. Look at the side panels to see whether the cushion becomes thinner towards the top.
A simple rectangular measurement may not describe a shaped back cushion accurately. If the cover has angled sides or a curved top, create a full size template of the front profile and note the required depth separately.
Back cushions may also contain loose fibre, foam crumb or shaped components rather than one flat foam block. Measuring alone does not confirm that a solid sheet is the correct replacement. Check the original construction before choosing the material.
How to measure bench and window seats
Bench seats and window seats often look rectangular but are frequently fitted into spaces that are not square.
Measure the width at the front, middle and back. Measure the depth at the left side, centre and right side. Check both diagonals from opposite corners. If the diagonal measurements differ, the opening is not a true rectangle.
Also check for skirting boards, wall projections, window frames, posts, hinges or handles that reduce the usable space.
For a loose cushion placed on top of a bench, measure the flat supporting area. For foam fitted inside a timber frame, measure the internal opening rather than the outside dimensions of the furniture.
Do not force a rectangular foam piece into a tapered recess. A full size template is more reliable when the front and back widths differ or when a wall curves.
How to make a template for shaped foam
A template records a shape that cannot be described safely with width and length alone.
Use firm paper, card or another material that will hold its shape. Place it in the finished position and trim it until it follows the required outline. Do not rely only on a photograph because an image does not preserve scale or show small changes in angle accurately.
Mark the top surface, front edge and left or right side. These labels prevent the template from being reversed.
Draw every cut out, curve and angled corner clearly. Mark whether the line represents the finished foam edge or an obstruction that needs clearance. Record thickness separately because a flat template only describes the plan shape.
Check the completed template in the actual space before using it to cut foam. It should sit flat without folding, forcing or leaving unexplained gaps.
For complex shapes or unusual mattress spaces, contact the All Foam team before ordering. The current online selectors price standard shapes and dimensions. A template may require a separate assessment.
How to measure foam thickness
Thickness should be based on the available depth and the job the foam must perform.
For a removable cushion, measure the cover side panel and review the complete insert construction. For a bench, measure the intended finished seat height and the clearance beneath tables, arms or surrounding fittings. For packaging, measure the space available above, below and around the item. For a mattress topper, consider the mattress depth and the fitted sheet depth together.
Do not measure only the deepest point of old compressed foam. Lay the original insert on a flat surface without applying pressure and compare it with the cover or opening.
Do not choose extra thickness simply because a deeper product appears more substantial. Too much depth can strain a zip, raise a seat beyond a comfortable height, prevent a case from closing or make a fitted sheet too shallow. Too little depth can leave loose fabric, expose a hard support or allow a protected item to move.
Thickness and firmness are separate choices. A thicker soft foam does not become the same product as a thinner firm foam. The foam density and firmness guide explains why grade, hardness and depth must be considered together.
How to measure packaging foam
Protective packaging requires measurements for the container and the item.
Measure the internal length, width and depth of the box, case or drawer. Internal dimensions matter because hinges, handles, ribs, lid linings and wall thickness can reduce the usable space.
Then measure the product at its widest, longest and deepest points. Include switches, connectors, feet, knobs and other projections. If accessories will share the case, measure them and decide where they will sit.
Plan the foam beneath the item, around its sides and above it. The correct thickness depends on item weight, fragility, expected movement and available case space. There is no single clearance that suits every product.
For a cut out insert, decide whether the item should sit fully below the foam surface or remain partly exposed for easy removal. Finger access, cables and removable parts may change the shape of the opening.
Check whether the lid already contains foam. The total depth of the base insert, item and lid foam must allow the case to close without crushing the item or leaving it loose.
The packaging foam guide explains how product movement, repeated handling, electronics and reusable cases affect material choice.
Measuring for closed cell foam and planned fixings
Closed cell sheets are often used for lining, sealing, protective inserts and gap filling. Measure the surface or opening that the finished piece must cover, then check the thickness available once the part is fitted.
For a lining panel, measure the usable face rather than the outside of the case. Deduct any channels, hinges or fittings that interrupt the surface.
For a gasket or seal, record the outside shape, inside opening and material thickness. Check how much compression the complete assembly requires. The product page does not replace the design requirements of the finished seal.
If a separate adhesive or fixing method will be used, decide which face will be fixed before cutting or fitting. A fixing system does not correct an inaccurate shape, uneven surface or unsuitable substrate. The surface should be assessed before installation.
The open cell and closed cell foam guide explains the different material families available for cushioning, sealing, lining and protection.
How to measure a mattress topper
Do not order a topper from the bed size name alone. Measure the mattress surface and compare it with the exact dimensions shown on the topper page.
Measure across the mattress width at the top, centre and bottom. Measure the full length from head to foot. The tape should remain flat across the surface rather than following the curve over a rounded edge.
The current mattress topper range lists exact dimensions for each option. Single toppers are 191 cm by 91 cm. Double toppers are 191 cm by 137 cm. King Size toppers are 198 cm by 152 cm. Super King toppers are 198 cm by 183 cm.
Also check the selected topper thickness. The memory foam topper is available in 2.5 cm, 5 cm and 7 cm depths. The standard foam and Reflex toppers are listed at 5 cm.
Add the topper depth to the existing mattress depth when checking fitted sheets, protectors, rails and headboard position. A topper changes the total sleeping surface height even though it does not change the mattress footprint.
If the mattress is deeply sagging, damaged or poorly supported, measuring for a topper does not solve the underlying problem. The mattress topper or new mattress guide explains when a surface layer is suitable and when replacement should be considered.
How to measure a standard mattress
Measure the bed base or internal frame before selecting a mattress.
Record the clear width and length available for the mattress. Check the space at several points if the frame is old, handmade or positioned between fixed furniture. Bed rails, brackets and upholstery can reduce the usable opening.
The current foam mattress range uses these exact dimensions.
Single measures 90 cm by 190 cm.
Double measures 135 cm by 190 cm.
King Size measures 150 cm by 198 cm.
Super King measures 180 cm by 198 cm.
Compare the frame measurement with the exact product dimensions. Do not assume that every bed described by the same name has identical internal measurements, particularly with older, imported or custom furniture.
Check mattress depth separately. The three standard All Foam mattresses have a total stated thickness of 8 inches, which is approximately 20 cm, including the cover.
Measure the height of surrounding bed rails and the position of the headboard. A mattress that sits too low may be difficult to access. A deeper mattress may cover more of the headboard or change the height of the sleeping surface.
Measuring for a custom foam mattress
The online custom foam mattress builder currently prices four standard UK widths and lengths with finished depths of 8, 9 or 10 inches.
Choose the standard size only after checking the exact centimetre dimensions against the frame. Then select the finished depth and internal layer construction.
For an unusual bed frame, caravan, campervan, motorhome or boat, record the finished width, length and preferred depth. Check every corner and measure diagonals. Built in spaces often taper or include curved walls and cut outs.
The online builder does not calculate non standard widths, lengths or shapes. Contact All Foam before ordering so the required measurements and supply options can be confirmed.
Common measuring mistakes
Copying old compressed foam
Old foam may be smaller, thinner or less square than it was originally. Use the cover, opening or supporting platform as the main reference.
Measuring outside dimensions
A case, frame or box can have thick walls. Measure the internal usable space when the foam must fit inside it.
Measuring only once
Furniture, walls and cases may not be square. Take several readings across large or fitted areas.
Stretching a fabric cover
Smooth the cover on a flat surface without pulling it. Stretching the fabric produces dimensions that the finished insert may not need to fill.
Adding an arbitrary allowance
Extra size may be appropriate for some covers, but it is not a universal rule. Consider the fabric, seams, wrapping and assembly method before changing the measured dimensions.
Ignoring the complete depth
The foam may share space with wadding, a mattress, lid foam, a rigid base or another layer. Measure the whole construction, not only one component.
Assuming every sheet has the same dimensions
Sheet sizes vary across upholstery, packaging, closed cell and acoustic products. Check the exact selector for the chosen foam.
Confusing size names with exact dimensions
Single, Double, King Size and Super King are useful labels, but the listed centimetre dimensions are the final reference for mattresses and toppers.
Rounding too early
Record the original measurement before converting units or choosing the nearest product option. Early rounding can create a larger difference across several parts.
Forgetting orientation
Mark the front, top and left or right side on shaped pieces and templates. This matters when a piece is not symmetrical.
A final check before ordering
Read every recorded dimension back in the same order: width, length, thickness.
Compare the measurement with the exact product size and thickness option. Confirm the foam grade suits the application. Check whether the piece is rectangular or needs a template. Review any cover, wadding, adhesive, lid foam or support layer that affects finished fit.
For multiple pieces, confirm that the chosen sheet can contain the complete cutting plan. For a topper or mattress, compare the exact centimetre dimensions with the bed rather than relying only on the size name.
Review the delivery and returns information before completing the order. Bespoke foam and customised products may not be exchangeable or refundable where the item is not damaged, defective or incorrect. Memory foam products also have specific return conditions for hygiene reasons.
If any measurement is uncertain, contact All Foam before ordering. A short check at this stage is more useful than trying to correct a size problem after a customised product has been supplied.
