How to Measure Stairs and Railings

What you need

A tape measure, a spirit level, a pencil and paper, and a helper — one person holds the tape at the top while the other reads it at the bottom. Have the stair calculator open: it turns your rise and tread into steps, run and angle.

Step 1 — Measure the total rise

Measure vertically from the finished floor below to the finished floor above. Measure in two places and use the larger number — floors are rarely level. A typical storey is about 2.8 m.

Step 2 — Divide into equal risers

Divide the total rise by your desired riser height (typically 175 mm) and round up to whole steps. Then divide the rise by the step count to get the actual riser. A 2.8 m rise at 175 mm needs ceil(2,800 ÷ 175) = 16 steps, each exactly 175 mm. Never mix riser heights on one flight — it is a trip hazard.

Step 3 — Work out the run

The total run is (steps − 1) × tread depth. The top step is the floor itself, so there is one fewer tread than steps. With 16 steps and a 250 mm tread, the run is 15 × 250 mm = 3.75 m. The stair calculator does all of this, including the overall slope angle.

Step 4 — Measure the railing

Measure the railing length along the slope (or use the run and rise to work it out). Balusters go every 100 mm at most in many codes — a 100 mm sphere must not pass through. Posts go about every 1.5 m. The railing calculator counts both: balusters = ceil(length ÷ spacing) + 1, posts = ceil(length ÷ post spacing) + 1.

Worked example

Total rise 2.8 m, riser 175 mm, tread 250 mm: 16 steps, actual riser 175 mm, run 3.75 m, stair pitch 35.0° (atan 175 ÷ 250). The overall slope of the flight is atan(2,800 ÷ 3,750) = 36.75° — that is the angle the stair calculator reports. A 5 m railing at 100 mm baluster spacing needs 51 balusters and 5 posts.

Common mistakes

• Unequal risers. The most dangerous stair mistake. Always divide the rise equally.
• Counting the top step as a tread. There is one fewer tread than steps — the floor is the last step.
• Spacing balusters too wide. 100 mm is the common maximum gap. Wider gaps are a child-safety risk and usually against code.

Safety

Stairs are safety-critical, and the rules exist because falls on stairs cause serious injuries. Keep every riser exactly equal — an uneven riser is a trip hazard. Keep baluster gaps to 100 mm or less where your code requires it, and make sure the handrail is fixed solidly: a loose rail is worse than no rail. Check your local building code for riser height, headroom and handrail rules before you build, and wear eye protection when cutting.

When to hire a professional

Hire a joiner, carpenter or stair specialist for a new staircase, for any structural change, or if your measurements do not meet your local code. Winder stairs (steps that turn a corner) and spiral stairs are also professional work — their geometry is easy to get wrong, and a survey costs far less than rebuilding a stair.

Frequently asked questions

• How much headroom do I need? About 2 m of clear headroom above the stair is the usual minimum — check your local building code for the exact figure. Measure it vertically from the nosing of each step to the ceiling above.
• Can I reuse my old railing? Only if it meets the current rules: check the baluster gaps and the handrail height against your local code first. If the gaps are wider than 100 mm or the rail is loose, replace it.
• What is a winder stair? A stair that turns a corner using wedge-shaped steps instead of a landing. Winders are harder to measure and to build safely — get a professional to survey one before you order.
• Do I count the landing as a step? No. A landing is a flat platform, not a riser. Measure each flight separately and treat the landing as its own flat area.

Try these calculators

• Stair Calculator — steps, riser, run and angle.
• Railing Calculator — balusters, posts and length.
• Door Size Calculator — door area and rough opening.