Calculator
Defaults apply when tolerance fields are blank.
Example data table
| Clear opening (W×H) | Side gap | Top gap | Bottom gap | Jamb thickness | Recommended slab (W×H) | Recommended rough opening (W×H) |
|---|---|---|---|---|---|---|
| 32 × 80 in | 1/8 in | 1/8 in | 3/4 in | 3/4 in | 33.75 × 81.625 in | 35.25 × 82.125 in |
| 900 × 2100 mm | 3 mm | 2 mm | 10 mm | 19 mm | 944.0 × 2131.0 mm | 982.0 × 2141.0 mm |
Formula used
slabH = clearH + topGap + bottomGap + jambT
roughH = slabH + headAllow + sillAllow
perimeter = 2·(slabW + slabH)
How to use this calculator
- Choose units to match your plans and measurements.
- Enter the required clear opening width and height.
- Adjust gaps and allowances to match your hardware.
- Click Calculate to view slab and rough opening sizes.
- Download CSV or PDF to attach to takeoffs.
- Verify sizing with your door supplier and code rules.
Door sizing guide for project documentation
1) Start from the required clear opening
Clear opening is the finished passage after the door is installed and opened. Residential interiors often aim near 28–32 inches clear, while accessible routes typically need more. Always measure from finished surfaces, not framing, and confirm where the stop and seals reduce effective clearance.
2) Use practical operational gaps
Consistent gaps prevent binding and uneven reveals. Many wood doors run well with about 1/8 inch on each side and at the top. Bottom clearance commonly falls around 1/2 to 3/4 inch, adjusted for rugs, airflow, and floor transitions.
3) Include jamb thickness in the geometry
Frame geometry matters because the jamb occupies width and height around the slab. A common jamb leg thickness is roughly 3/4 inch, but thicker stock or metal frames can increase it. Enter the actual value so the slab maintains the clear opening you need.
4) Allow for shims and installation tolerance
Rough openings must accommodate shimming and plumb correction. A typical allowance is about 1/4 inch per side plus a small head buffer. A widely used check is rough width ≈ door width + 2 inches and rough height ≈ door height + 2 inches. Record site conditions and confirm framing is square.
5) Adjust for exterior thresholds and weathering
Exterior doors often require thresholds, sweeps, and tighter water control. Bottom gaps can differ from interiors, and outswing doors may need different sill detailing. Use sill allowance when a threshold build-up raises the finished floor line relative to the frame.
6) Use standard sizes to reduce lead time
Common slab widths include 24, 28, 30, 32, 34, and 36 inches. Typical heights are 80 and 84 inches, with 96 inches for taller designs. Comparing to standard inventory helps avoid custom fabrication, pricing premiums, and schedule delays.
7) Metric inputs with realistic clearances
For metric work, typical gaps may be 3 mm per side, 2 mm at the top, and 8–12 mm at the bottom. A 900 × 2100 mm clear target often yields a slab near 944 × 2131 mm with common jamb assumptions.
8) Perform final checks before ordering
Confirm final flooring thickness, hinge side, swing, and hardware backset before you release a purchase order. Fire-rated, acoustic, and specialty doors may have manufacturer-specific clearances that override defaults. Export CSV or PDF so takeoffs remain consistent across teams.
FAQs
1) What is the difference between door slab and rough opening?
The slab is the door leaf size. The rough opening is the framed hole that allows the frame, shims, and adjustments so the installed door can be plumb, level, and square.
2) What side gap should I use for smooth operation?
Many interior doors work well around 1/8 inch per side. Tight gaps can bind with seasonal movement, while overly large gaps look unfinished and may reduce privacy.
3) How do I account for carpet or thick flooring?
Increase the bottom gap to prevent rubbing after flooring is installed. If the floor build-up is unknown, plan conservatively and verify on site before final trimming or ordering.
4) Are exterior doors sized differently?
Often, yes. Thresholds, weather seals, and water management details affect bottom clearance and sill height. Use the sill allowance and confirm requirements with the door manufacturer.
5) Why is the nearest standard size only a reference?
Standard offerings vary by supplier and region. The reference helps align estimates with common inventory, but the calculated slab and rough opening should drive final specifications.
6) Should I always add exactly 2 inches to the door size for rough opening?
It’s a common rule of thumb, not a universal rule. Frame type, shimming needs, and tolerance targets can change it. This calculator lets you set your own allowances.
7) Does swing direction change the numeric sizing?
Swing mostly affects detailing, not basic geometry. However, outswing doors may need different sill details and weathering, so review bottom clearance and thresholds carefully.