Metric stair calculator

Metric stair layout uses the same relationships as imperial stairs, just in millimetres and metres instead of inches and feet: risers = round(total rise ÷ target riser), riser height = total rise ÷ risers, going = tread depth, and total run = treads × going.

A typical floor-to-floor height in a metric-drawn house is around 2700 mm, though this varies by region and floor construction just as much as imperial floor-to-floor heights do. Always take your actual measurement rather than assuming a standard.

Riser height on a metric domestic stair is commonly cited in the 170 to 180 mm range, and going (the metric term for tread depth) in the 250 to 280 mm range. These are typical, commonly seen figures, not universal minimums or maximums, since exact limits depend on the building regulations that apply where you're building.

A taller riser paired with a shorter going gets you up a floor in fewer steps but feels steeper; a shorter riser with a deeper going feels gentler but takes up more floor area. The Blondel check below is the usual way to balance the two.

Blondel's comfort formula translates directly: 2R + G, where R is riser height and G is going, both in millimetres. It's commonly targeted at roughly 600 to 650 mm total, aiming for a stride that matches a natural walking pace on the flat.

For a 180 mm riser and 270 mm going: 2(180) + 270 = 630 mm, which sits comfortably inside that range.

If you're translating between an imperial plan and a metric one, or checking a metric result against imperial reference numbers, these are the conversions you'll reach for most often.

Once you've calculated an exact riser height, round it to a whole millimetre for the layout, and don't try to hold a tolerance tighter than about 1 mm between risers, since that's below what a tape or story pole can reliably repeat on site.

The commonly cited variation limits between adjacent risers in imperial code language (around 10 mm, roughly equivalent to the ⅜ in figure used in the US) are a useful target to hold yourself to even where local rules are phrased differently.

1. Calculate exact riser height from total rise ÷ number of risers.
2. Round to the nearest whole millimetre for the story pole and stringer layout.
3. Cut a test riser and confirm it against the story pole before cutting the full set of stringers.

Metric-market stair stock is typically sold in round millimetre widths and thicknesses rather than nominal lumber sizes: treads commonly come in 32 mm or 40 mm finished thickness, and stringer or string material is often supplied in 250 to 300 mm widths to give enough depth for a full-height cut string.

If you're sourcing from an imperial supplier for a metric-designed stair, convert your target board sizes to the nearest stocked imperial size rather than assuming an exact millimetre match is available off the shelf.