Fitting an Internal Door: The Complete DIY Guide
Fitting a new internal door is one of those DIY jobs that looks intimidating but is genuinely achievable with basic tools and a bit of patience. Whether you're replacing a single door or fitting out an entire house, this step-by-step guide covers everything you need.
Tools You'll Need
- Tape measure
- Pencil
- Spirit level
- Chisel (25mm)
- Mallet
- Hand plane or electric planer
- Drill/driver
- Screwdriver
- Workbench or saw horses
- Fine-tooth hand saw or circular saw
- Sandpaper (120 grit)
Step 1: Measure Your Existing Door
If you're replacing an existing door, measure it precisely:
- Measure the height at both sides of the door (left and right edges)
- Measure the width at the top, middle, and bottom
- Measure the thickness — standard is 35mm (internal) or 44mm (fire doors)
- Take the smallest measurements — these are your maximums
If you're measuring an empty frame:
- Measure the frame opening, then subtract 6mm from width (3mm gap each side)
- Subtract 8mm from height (3mm top gap + 5mm bottom clearance, or more for carpet)
Step 2: Choose Your Door
Standard UK internal door sizes:
| Imperial | Metric (mm) | Common Name |
|---|---|---|
| 6'6" × 2'0" | 1981 × 610 | Small/cloakroom |
| 6'6" × 2'3" | 1981 × 686 | Narrow |
| 6'6" × 2'6" | 1981 × 762 | Standard |
| 6'6" × 2'9" | 1981 × 838 | Wide |
Browse our full internal doors collection for all sizes and styles.
Step 3: Trim to Fit
Most doors need slight trimming:
- Mark the amount to remove with a pencil line
- Score the line with a sharp knife to prevent splintering
- Cut with a fine-tooth saw — use a straight edge as a guide
- Plane the edges smooth with a hand plane
- Important: Never remove more than the manufacturer recommends (usually 10mm per edge). This is especially critical for fire doors where over-cutting voids the fire rating.
Step 4: Mark and Cut Hinge Recesses
- Place the door in the frame with wedges to set correct gaps
- Mark hinge positions — standard is 150mm from the top and 225mm from the bottom
- For heavy doors or fire doors, add a third hinge centrally
- Transfer marks to both door edge and frame
- Use the hinge as a template to mark the recess depth and width
- Chisel out the recess to the exact hinge thickness — it should sit flush
Step 5: Fit the Hinges
- Screw the hinge into the door edge first (pilot holes prevent splitting)
- Offer the door into the frame with the hinge pins aligned
- Drive one screw per hinge into the frame to test the fit
- Check gaps and swing — adjust if needed
- Once happy, drive remaining screws
Choose from our range of quality door hinges — we stock butt hinges, ball bearing hinges, and fire-rated options.
Step 6: Fit the Latch and Handle
- Mark the handle height — standard is 1000mm from floor level
- Drill the spindle hole through the door (check handle instructions for size)
- Drill the latch hole into the door edge
- Chisel out the latch faceplate recess
- Insert latch, fix faceplate, attach handles both sides
- Mark and chisel the strike plate position on the frame
Browse our door handles and locks & latches for the complete selection.
Step 7: Final Adjustments
- Check the door swings freely without catching
- Verify even gaps all around (2-3mm sides and top, 5-10mm bottom)
- Test the latch engages smoothly
- Sand any rough edges
- Apply finish — paint for white primed doors, oil or varnish for oak doors
Fitting a Fire Door? Extra Steps
Fire doors have additional requirements:
- Use three fire-rated hinges (not two)
- Fit intumescent strips in the door edge or frame
- Use fire-rated hardware only
- Maximum gap of 3mm around all edges
- Fit a self-closer if required (commercial/HMO)
- Never cut more than the manufacturer allows
Common Mistakes to Avoid
- Not acclimatising the door: Leave it in the room for 48 hours before fitting to adjust to humidity
- Cutting from the wrong side: Veneer doors can chip — always cut from the face side
- Hinge recesses too deep: This causes the door to bind. Better too shallow than too deep.
- Skipping pilot holes: Hardwood and MDF will split without pilot holes
- Ignoring the frame: If the frame is twisted or out of square, fix that first
Need Help?
If you're not confident with DIY, a joiner can typically hang an internal door in under an hour. Either way, choosing quality doors makes the job easier and the result better. Browse our complete door range with express shipping on all UK orders.