Replacing cushion foam? Measure the cover, not the old insert. Old foam can shrink, sag and spread, so copying it often gives you a new cushion that feels loose before it has even been used.
Take the old foam out, lay the cover flat, measure seam to seam, then enter those numbers in the foam cut to size calculator. Add a little fullness only when the cover has enough give to take a tighter insert.
The 30-second version
- Remove the old foam before measuring.
- Zip or close the cover and smooth it flat without stretching.
- Measure width, depth and thickness from seam to seam (adding 10mm to width and depth for a tight snug fit)
- Use millimetres when ordering custom cut foam online.
- Do not reduce the foam thickness size just because you add Dacron wrap as this is very squishy and should not physically change the overall height.
What Should You Measure?
| Project | Measure this | Watch out for |
|---|---|---|
| Sofa seat cushion | Cover width, depth and side-panel height | Old foam that has flattened through the middle |
| Back cushion | Cover width, height and thickness | Pulling the cover larger than its natural shape |
| Dining chair pad | Seat board or cover shape | Guessing from bulky fabric edges |
| Custom or odd shape | Nearest calculator shape plus a sketch or notes | Forcing a curved or notched cushion into a plain rectangle |
Why the Old Foam Is a Bad Measuring Guide
Old cushion foam rarely keeps its original size. It can lose height, bow at the front, crumble near the zip or spread sideways inside the cover. If you measure that worn insert, you are measuring the problem.
The cover is the shape the new foam has to fill. That is why the cover should be your main guide for replacement couch cushion foam.
How to Measure the Cover
- Remove the old insert. If it is breaking down, take it out slowly so old foam dust stays contained.
- Close the cover. Zip it up or sit the opening as naturally as possible.
- Lay it flat. Smooth the fabric with your hands, but do not stretch it.
- Measure seam to seam. For boxed cushions, take width, depth and side-panel height (thickness).
- Measure across the centre too. Corners and piping can make edge measurements misleading. Generally take the largest overall dimension so it fits back in snug.
- Write the numbers in millimetres (mm). That keeps them ready for the custom foam calculator. Handy Tip: If you are used to measuring in centimetres (cm) add a 0 to your dimensions eg 53cm this would equal 530mm.
Should You Add Extra for a Fuller Cushion?
Yes, most of the time. Generally a small 10mm (1cm) allowance added to the width and depth can help a cushion look plumper and fuller, but only if the cover fabric, seams and zip can handle the extra pressure.
Skip the allowance if the cover is old, brittle, heavily piped, already tight or hard to zip. A cushion that is slightly snug is workable. A cushion that is too large can strain the cover and make the insert difficult to fit.
What About Dacron Wrap?
Dacron is a finish layer, not a sizing correction. It rounds the edges, helps the cushion fill the cover cleanly and makes the insert easier to handle. Keep the core foam measurements based on the cover.
For more detail, read the Dacron wrap and foam overlay guide.
Odd Shapes, Curves and Templates
If your cushion is close to a standard shape, choose that shape in the calculator and add notes for anything unusual. L-cushions, T-cushions, wedges and trapeziums often fit this workflow well.
If the cushion has curves, notches, angled corners or has to match a frame closely, prepare a sketch or template. Mark the front, back, left, right and thickness so the cutting team can understand the orientation.
Before You Order
- Replacing a whole lounge? Consider ordering one cushion first if the fit is uncertain.
- Changing the cushion thickness? Read how to choose foam thickness before you order.
- Unsure which grade to choose? Use the foam grade and density guide.
- Adding a wrap or overlay? Check how wrap or overlay affects cushion sizing before changing measurements.
- Need general product options? Browse foam and fillings.
Once your measurements are ready, start your custom foam order, choose the closest shape, enter the dimensions, select the application and compare suitable grades.
Measuring Cushion Covers FAQs
Do I measure the old foam or the cushion cover?
Measure the cover. Old foam can be compressed, shrunken or distorted, so it is not reliable for final sizing.
Should I measure inside or outside the cover?
Measure the cover seam to seam while it lies flat and unstretched. For most replacement sofa cushions, this gives the best practical size for the new insert.
Should I add 10mm to every measurement?
Not always. Add a small fullness allowance only when the cover construction can take it and you want a tighter, plumper fit.
Do I reduce the foam size if I add Dacron?
No. Dacron compresses around the foam and is normally added without reducing the core foam measurement.
What if my cushion is a custom shape?
Use the closest calculator shape only if it genuinely matches the job. For curves, cut-outs or unusual angles, use custom shape, and attach a sketch or template, and include notes.