# How to measure a clear phone case for an insert

**Short answer:** take the phone out, lay the case flat, and measure the *inside* of the back panel in millimetres: the width at three heights and the height at three widths. Use the smallest of each, subtract 0.5 mm, measure the camera opening from the nearest side wall and the top wall, and enter the result as a custom size.

By Matt Warcholinski · Published 21 October 2026 · 5 min read

A clear phone case with a metric ruler, a pencil, a sheet of paper and calipers

Illustration: what you need to measure a case.

**Key takeaways**

- Measure the case window, not the phone. The case decides what fits.
- Case walls are rarely perfectly straight: measure in three places and use the smallest number.
- Subtract 0.5 mm (1 mm for soft or thick cases) so the insert slides in without catching.
- Print one test sheet on plain paper before you use good paper.

On this page

1. [When you need to measure](https://phonecaseinserts.com/diy/measure-clear-phone-case#when-you-need-to-measure)
2. [What you need](https://phonecaseinserts.com/diy/measure-clear-phone-case#what-you-need)
3. [Measure it in six steps](https://phonecaseinserts.com/diy/measure-clear-phone-case#measure-it-in-six-steps)
4. [A worked example](https://phonecaseinserts.com/diy/measure-clear-phone-case#a-worked-example)
5. [Enter the size and test](https://phonecaseinserts.com/diy/measure-clear-phone-case#enter-the-size-and-test)
6. [Common mistakes](https://phonecaseinserts.com/diy/measure-clear-phone-case#common-mistakes)
7. [A trace method (no callipers)](https://phonecaseinserts.com/diy/measure-clear-phone-case#a-trace-method-no-callipers)
8. [Questions](https://phonecaseinserts.com/diy/measure-clear-phone-case#questions)

## When you need to measure

Start with the model template. It uses the manufacturer’s published size of the back of the phone, and the Fit setting in the maker covers small differences. Measure your own case when:

- your phone is not in the [size table](https://phonecaseinserts.com/phone-case-insert-size);
- the case has thick walls or a raised lip that makes the window smaller than the phone;
- a template still fits badly after you [print it at 100%](https://phonecaseinserts.com/diy/print-phone-case-insert-at-100-percent) (check that first: it is the most common cause);
- the camera opening is bigger than the phone’s camera, as on many MagSafe cases.

## What you need

- A metric ruler with millimetre marks. A digital calliper is better and costs little, but a ruler is enough.
- A pencil and a sheet of paper for notes.
- The clear case with the phone and any old insert removed.

## Measure it in six steps

Diagram: measure inside the case, not around it. The camera opening is measured as width, height, a (from the left edge) and b (from the top edge).

1. **Prepare.** Remove the phone and any old insert. Lay the case on a table with the open side up and the camera end at the top. Seen from the inside, the hole looks mirrored: it is on the right if the camera is on the left of the back.
2. **Width.** Measure the clear space from the inside of one side wall to the other at three heights, W1 to W3. Write all three numbers.
3. **Height.** Measure from the inside of the top wall to the inside of the bottom wall at three places, H1 to H3.
4. **Camera opening.** Measure the width and height of the hole, then the distance from the side wall nearest the hole to the hole’s near edge (a) and from the top inside wall to the hole’s top edge (b). For separate lenses, measure the box around all of them, or each hole if the openings are separate.
5. **Pick the smallest.** Use the smallest width and the smallest height. A slightly bowed wall in the middle is common, and the smallest number is the one that decides whether the paper catches.
6. **Subtract the gap.** Take 0.5 mm off each. Use 1 mm for soft or thick cases. Fit in the maker is measured from the phone’s published size; this gap is taken from your measured case window, so do not apply both.

## A worked example

An example only: say you measure a case for a phone that is 71.5 × 149.6 mm on the back, and the inside of the case reads like this.

| Measurement | Reading (mm) | Used |
| --- | --- | --- |
| Width W1, W2, W3 | 70.8, 70.5, 71.0 | 70.5 (smallest) |
| Height H1, H2, H3 | 149.2, 149.0, 149.1 | 149.0 (smallest) |
| After subtracting 0.5 mm |  | **70.0 × 148.5** |
| Camera opening | 38 × 38 at a = 5, b = 5 | use as a guide to place the window |

The case window is 1.0 mm narrower than the phone’s published width. That is exactly the kind of difference that makes a template for the phone bend inside the case.

## Enter the size and test

1. Open the [insert maker](https://phonecaseinserts.com/create), choose the closest phone model, then switch to a custom size and type your width and height.
2. The maker has a “Camera hole (mm, optional)” field: enter the opening’s size, then check the window position on the preview against your a and b measurements.
3. Print one test copy on plain paper using the [100% settings](https://phonecaseinserts.com/diy/print-phone-case-insert-at-100-percent), cut it and try it in the case.
4. If it catches, take 0.5 mm off. If it rattles, add 0.5 mm back.
5. Keep your final numbers in your notes app, with the case name, for next time.

## Common mistakes

- **Measuring the outside of the case.** The walls add several millimetres. Measure the clear space inside.
- **Measuring only once.** Walls bow slightly. Three readings take a minute.
- **Forgetting the printer.** If the printer scales the page by even 3 to 6%, your careful measurement is lost. Check the ruler on the sheet first.
- **Measuring a case with the lip curved inward.** Measure where the paper will actually sit, on the flat back panel, not across the curved rim.

## A trace method (no callipers)

Put the case back side down on a sheet of paper and draw round the outside with a pencil. Draw a second line inside the first, as far in as the thickness of the case wall. Cut along the inner line and test the paper in the case. Then measure the finished pattern with a ruler, or lay it on the printed 5 cm bar, and enter those numbers as a custom size.

A clear phone case lying on a sheet of white paper with scissors and a wooden ruler beside it

Illustration: a case on paper, ready to trace and cut.

## Questions

Should I measure the phone or the case?

The case. The phone’s own size is only the starting point for templates; the case window decides what fits.

Why subtract 0.5 mm?

An insert exactly the size of the window catches on the lip and bends. A small gap lets it sit flat. Increase it to 1 mm for thick or soft cases.

What if my case has a camera bar or raised ring?

Measure the opening in the back panel, not the raised ring around it. The ring sits outside the insert. See the [camera-hole guide](https://phonecaseinserts.com/diy/how-to-cut-the-camera-hole).

Will the same size work for other cases of the same phone?

Not always. Clear cases differ by 1 to 2 mm between makers, so keep the size with the case you measured.

Is a calliper worth buying?

It is more precise and easier to read, but a ruler gets you to within half a millimetre, which is enough for a paper insert.

**What this guide does not cover**The example numbers are made up to show the method. We have not measured specific cases here; the manufacturer sizes on our template pages are for the phone, not for any case. Printer shrink of 3 to 6%, case differences of 1 to 2 mm and a ruler’s half-millimetre accuracy are rough rules of thumb, not measurements.

## More guides

### Paper inserts in a MagSafe clear case

Will paper block MagSafe or wireless charging? What to check.

### How to make a phone case insert

Five steps from template to finished insert.

### Draw your own phone case insert (a kids' craft)

A kids' craft: draw it, cut it, slide it into a clear case.

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Source: https://phonecaseinserts.com/diy/measure-clear-phone-case
Agent index: https://phonecaseinserts.com/llms.txt