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.
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, and on a fire door is the part doing the work.
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 We deliberately do not publish a millimetre figure, since a wrong one gets a door cut past its lipping and thrown away.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.
Can you trim a fire door?
Usually a little, and only within the allowance the manufacturer states. Going beyond that voids the fire rating and the certification of the doorset it belongs to. Fire door allowances are product-specific and normally much tighter than an ordinary door, and some edges may not be trimmed at all. The fitting instructions are the only authority, and building control will ask for the certification.
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.