To take an inside measurement with a tape measure, press the end of the case into one side, extend the tape to the opposite side, read the number, and add the case length printed on the tool. That extra step is what separates a good inside reading from a wrong one. Here is the full process.
Why Inside Measurements Are Different
An outside measurement hooks the tape on an edge and pulls straight. An inside measurement has no edge to hook, so you push the case against one surface and read the tape at the other. The case body adds length that the tape alone does not show. Most tapes print the case width on the housing, often as a small number you add.
Because makers differ, read the case marking on your own tape or check the instructions that came with it.
Step-by-Step Method
- Find the case length on the tape housing, usually printed on the side or the back.
- Press the back of the case flat against one inside surface, such as a wall or cabinet side.
- Pull the tape across to the opposite surface, keeping it straight and level.
- Lock the tape, or hold it firmly in place with your thumb.
- Read the number where the tape meets the far surface.
- Add the case length to that reading.
- Write the total down at once, and note what it is for.
Another Method: The Hook Trick
If the space is narrow, you can place the hook against one side and read the tape at the opposite side, then measure the gap in a second step. Another approach is to bend the tape slightly in a corner and read a point on it. This is less accurate, so use it only when needed and double-check the result.
Ways to Improve Accuracy
- Measure at the top, middle, and bottom of the space, since walls and frames are rarely perfectly parallel.
- Keep the tape level and straight, because a sag adds error.
- Use the smallest of your readings if a piece must fit inside.
- Measure twice, and have someone else check if the cut is expensive in time or material.
| Mistake | Result | Fix |
|---|---|---|
| Forgetting the case length | Reading is too short | Add the printed number |
| Case not flat to the wall | Reading is off | Press the case firmly |
| Tape sagging | Reading is too long | Support or shorten the span |
| Measuring once only | Missed variation | Take three readings |
FAQ
Do all tape measures have the same case length?
No. The length varies by model, so use the number printed on your tape.
Why does my inside measurement not match a story pole?
Small errors from sag, a tilted case, or an uneven wall can add up. Re-measure, and consider a stick or rod cut to the gap for a final check.
The Short Version
Press the case flat against one side, extend to the other, read the tape, and add the case length printed on the tool. Measure in several places, keep the tape straight, and write the result down. Check the maker’s instructions for your tape.