---
title: "How to Measure Back Cushion Height: Subtract the Seat First"
url: "https://cushionguide.com/answers/back-cushion-height-subtract-seat-thickness/"
section: "The Cutting Table"
author: "Callum Reid, The Weather Man of Casco Bay"
author_url: "https://cushionguide.com/masters/callum/"
published: "2026-09-05"
updated: "2026-09-05"
keywords: "how to measure back cushion height, back cushion height minus seat thickness, backrest cushion size, futon back cushion measurements, built-in bench back cushions"
---

# Do I subtract the seat cushion thickness when measuring for back cushions?

*You do. Skip it, and a new set of cushions spends its first season shoving itself around like two boats tied too close.*

Also asked as: Does the thickness of the seat need to be deducted?; Height of back minus the cushion depth?

> **The quick answer:** Yes: measure the total back height, then subtract the seat cushion thickness, because the backrest sits on top of the seat cushion. With a standard 4 in (10 cm) seat, a 21 in (53 cm) back area takes a 17 in (43 cm) backrest. Skip the deduction and the seat and back cushions push each other out of place.

![Amber self-piped outdoor back cushion leaning on a light gray garden sofa beside terracotta pots](https://cushionguide.com/media/backrest-cushions/amber-outdoor-back-cushion-gray-garden-sofa-1600.webp "A single amber back cushion brings warmth to a cool gray patio sofa and picks up the terracotta pots behind it.")

## Key facts

- Backrest height = total back height minus seat cushion thickness.
- The standard seat cushion thickness is 4 in (10 cm).
- With a 4 in seat, a 21 in (53 cm) back area needs a 17 in (43 cm) backrest.
- Without the deduction, seat and back cushions push each other out.
- The same rule applies to futons and built-in seats.
- A quick sketch helps confirm the final sizes.

## Back height minus seat: worked examples

| Back area measured | Seat cushion | Backrest to order |
|---|---|---|
| 21 in (53 cm) | 4 in (10 cm), standard | 17 in (43 cm) |
| 22.5 in (57 cm) | 4-5 in (10-13 cm) | About 17.75 in (45 cm), as one buyer chose |
| 21 in (53 cm) | 5 in (13 cm) | 16 in (41 cm) |
| Any height | 4 in (10 cm) | Take 4 in (10 cm) off |

## Why the seat takes a bite out of the back

A backrest doesn't start at the frame. It starts where the seat cushion ends. Lay a 4 in seat down and the first 4 in of your back area is already spoken for.

Order the full height anyway and the back cushion has nowhere to go. It rides up, tips forward, or shoves the seat toward the front edge. Every time someone sits down, the two pieces argue, and nobody wins.

One buyer measured a 22.5 in (57 cm) back area, learned the seat would be 4-5 in thick, and cut the backrest order to about 17.75 in (45 cm). That's the whole trick, friend.

## Measure it in four moves

1. **Measure the back**: Take the total back height from the frame, from the seat deck up to where you want the cushion to stop.
2. **Find the seat thickness**: Use the thickness you are actually ordering. Standard is 4 in (10 cm); some floor couches run 4-5 in.
3. **Subtract**: Back height minus seat thickness is your backrest height.
4. **Sketch and send**: Draw the seat and back with every number on it. The workshop confirms the final sizes from the sketch.

## Futons and built-ins play by the same rule

The rule holds on a futon frame or a built-in bench. One futon set was a 23 x 66 in (58 x 168 cm) seat with two 23 x 33 in (58 x 84 cm) backrests, side pillows included.

On the patio it matters twice: [outdoor cushions](https://cushionguide.com/cushions/outdoor-cushions/) that fight each other hold water in the gap. The [back cushion height Pattern](https://cushionguide.com/guides/back-cushion-height-sizing/) has more layouts, and the [backrest cushions](https://cushionguide.com/cushions/backrest-cushions/) page shows the shapes. What does your back area measure at the seat line?

> **Tailor's margin:** Back cushion height is the back area minus the seat cushion: a 21 in back over a 4 in seat takes a 17 in backrest, not 21.

> Measure back cushions from the top of the seat cushion, not the frame; a 4 in seat takes the first 4 in of back height.
>
> — Callum

## Buyer questions

**A common question:** Sizing backrests to sit above a seat cushion: do I take off the seat thickness?

**The workshop's answer:** Yes. The seat cushion's thickness, for example 4 in (10 cm), comes off the backrest height so the pieces fit without pushing each other out. A quick sketch helps confirm the final sizes.

**A common question:** How should I measure the back cushion height on my futon or built-in seat?

**The workshop's answer:** Measure the total back height from the frame, then subtract the seat cushion thickness, standard 4 in, because the seat cushion takes up part of the back. One futon set was a 23 x 66 in seat with two 23 x 33 in backrests, side pillows included.

**A common question:** Should the seat cushion thickness come off the wall height when I measure for a backrest?

**The workshop's answer:** Yes. The backrest sits on top of the seat, so deduct its thickness: with a 4 in seat, a 21 in back area needs a 17 in backrest. One buyer with a 22.5 in (57 cm) back area reduced the backrest to 17.75 in (45 cm) after learning the seat would be 4-5 in thick.

**A common question:** What do you need from me to make seat and back cushions for my chairs?

**The workshop's answer:** The width between the armrests, the seat depth and the back height you want, plus a photo of the chair. Overall chair dimensions alone aren't enough for a good fit.

## Questions buyers ask next

### Does the rule change if my seat cushion is thicker than 4 inches?

Only the number changes. Subtract your actual seat thickness: a 5 in seat under a 21 in back area leaves a 16 in backrest. Always use the thickness you are ordering, not the standard figure.

### What goes wrong if I order the backrest at the full back height?

The back cushion has no room. It sits on the seat cushion, runs too tall for the space, and the two pieces push each other out of place.

### Do back cushions need to be as thick as the seat?

No. Many sets pair a 4 in seat with a 2 or 3 in back; state each thickness separately. Keep a foam backrest at 2 in (5 cm) or more, since thinner foam flattens against the back.

## Related at the atelier

- [Should back cushions be the same thickness as the seat cushions?](https://cushionguide.com/answers/back-cushion-thinner-than-seat-cushion/): Back cushions don't have to be as thick as the seat cushions.
- [Can you make banquette seat and back cushions for a built-in breakfast nook?](https://cushionguide.com/answers/banquette-seat-and-back-cushions-breakfast-nook/): Yes, banquette seat and back cushions for a breakfast nook are both made to measure: send the size of each seat run and each back section.
- [How tall should a back cushion be? Sizing backrests to your seat](https://cushionguide.com/guides/back-cushion-height-sizing/): A standard back cushion is 16 in (40 cm) tall and 4 in (10 cm) thick, made the same length as the seat, with its height measured from the top of the seat cushion.
- [How do I measure backrest height: from the top of the seat cushion or from the bench?](https://cushionguide.com/answers/how-to-measure-backrest-cushion-height/): Measure backrest height from the top of the seat cushion, so the backrest sits on the bottom cushion; the shop usually recommends this because it looks and fits better.
- [Reading Wedge Backrest in Charcoal for a Bed Shared with a Dog](https://cushionguide.com/readings/charcoal-reading-wedge-dark-green-bedroom-whippet/): A firm triangular reading wedge gives upright back support in bed without replacing your pillows, and it lasts when the finishing is right: piping that follows the slope, side panels that stand flat…
- [Bench Seat and Back Cushion in Jacquard and Terracotta Bouclé](https://cushionguide.com/readings/geometric-backrest-terracotta-boucle-seat-closeup/): For a bench seat and back cushion in two fabrics, put the forgiving cloth on the seat and the pattern on the back.
- [Replacement cushions for a nursery glider, piece by piece](https://cushionguide.com/guides/glider-rocker-replacement-cushions/)

*Fair winds — Callum*

---

Written by Callum Reid (https://cushionguide.com/masters/callum/) for CushionGuide (https://cushionguide.com/about/). The site explains made-to-measure cushions and the partner workshop makes them; measurements are in inches first. This Markdown mirrors the page at https://cushionguide.com/answers/back-cushion-height-subtract-seat-thickness/