# Door trimming calculator — what can come off | Threshen

> Work out how much has to come off each edge to fit a standard door into your opening, and check it against the trimming allowance your door manufacturer states.

Source: https://example.co.uk/tools/trimming-calculator

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# Trimming calculator

How much timber has to come off each stile and off the bottom to fit a standard door into your opening — and whether that is more than the door is allowed to lose.

We do not supply the allowance — you do

There is no default trimming limit in this tool. The allowance belongs to the manufacturer of your specific door, and inventing a plausible-looking number here would get doors cut past their lipping. Type in the figure from your fitting instructions and the calculator will check your measurements against it; leave it blank and it will still do the arithmetic, but it will not pretend to judge the result.

## What has to come off

| Edge | Door | Opening | Trim required |
| --- | --- | --- | --- |

## Why there is a limit at all

### The lipping

A veneered or moulded door usually has a solid timber lipping around the edge, protecting the core and giving the hinges and latch something to bite into. Trim past it and you are into the core — which on a hollow-core door is not much at all.

### Take it off evenly

Trim is normally shared between both stiles rather than taken off one, so the stiles stay the same width and the door still looks right — and so neither side loses more lipping than it can spare. This tool splits the width figure for you.

### Height comes off the bottom

Height is usually taken off the bottom rail rather than the top, because the top of the door is what you see and the bottom is where the floor clearance lives. Check the fitting instructions before assuming, and check whether the door has a lock block that fixes which way up it hangs.

## Common questions

### How much can I trim off a door? +

It depends entirely on the door. The allowance is set by whoever made it and printed in the fitting instructions, because it is governed by the lipping and the core underneath. We deliberately do not publish a millimetre figure, since a wrong one gets a door cut past its lipping and thrown away. Enter your own figure in the calculator and it will check your measurements against it.

### Do I take the height off the top or the bottom of the door? +

Height normally comes off the bottom rail — the top of the door is what you see, and the bottom is where floor clearance lives. Width is normally shared between both stiles rather than taken off one, so the door stays symmetrical and neither edge loses more lipping than it can spare. Check the fitting instructions first: some doors have a lock block that fixes which way up they hang.

## Carry on

- [How to measure a door opening](https://example.co.uk/advice/how-to-measure-a-door)
- [UK door size guide](https://example.co.uk/advice/door-sizes)
- [How much can you trim off a door?](https://example.co.uk/advice/how-much-can-you-trim-a-door)