What this calculator works out
Enter the floor-to-floor height, the number of risers you intend and the going you want, and this works out the rise per step, the pitch, the comfort ratio and how much floor space the flight will occupy. It then checks all of it against the limits in Approved Document K for a private stair.
Stairs are among the few pieces of domestic construction where the dimensions are prescribed rather than advisory, and for good reason: falls on stairs are one of the most common causes of serious injury in the home.
This is a design check to help you arrive at a workable geometry before you talk to Building Control. It is not approval, and it does not check headroom, width, handrails or guarding.
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Before you calculate
- Measure floor-to-floor, from the finished surface downstairs to the finished surface upstairs. If flooring is still to be laid, allow for its thickness at both ends.
- There is always one more riser than tread, because the top riser lands on the upper floor.
- Every rise in a flight must be equal, and every going must be equal. Uneven steps are the most common cause of stair falls.
- This checks a private stair, meaning one serving a single dwelling. A stair shared between flats is a common stair and must meet stricter limits.
Building work on stairs almost always needs Building Regulations approval, and only your local Building Control body or an approved inspector can confirm compliance. Use this to sketch a workable geometry before you speak to them, not instead of speaking to them.
How this calculator works
The rise is simply the height divided by the number of risers. The pitch comes from the ratio of rise to going:
Rise = total height ÷ number of risers
Pitch = arctan(rise ÷ going)
Comfort rule: 2 × rise + going
Total run = going × (risers − 1)It then checks the result against the limits in Approved Document K for a private stair serving one dwelling:
| Measure | Limit | Why it exists |
|---|---|---|
| Maximum rise | 220 mm | Taller steps are tiring and easy to catch a toe on |
| Minimum going | 220 mm | Shallower treads do not support enough of the foot |
| Maximum pitch | 42° | Steeper stairs become difficult to descend safely |
| 2 × rise + going | 550 to 700 mm | The relationship that makes a stair feel comfortable to walk |
| Minimum headroom | 2,000 mm | Measured vertically from the pitch line; 1,900 mm is allowed at the edge in some loft conversions |
| Maximum risers in a flight | 16 | Beyond this a landing is required to break the fall distance |
The comfort rule matters more than people expect. A stair can satisfy every individual limit and still feel wrong underfoot if the rise and going are badly matched, which is what the 2R+G range is designed to prevent.
Worked example: a 2.6 m floor height
A floor-to-floor height of 2,600 mm with 13 risers and a 230 mm going:
- Rise: 2,600 ÷ 13 = 200 mm — within the 220 mm maximum
- Going: 230 mm — above the 220 mm minimum
- Pitch: arctan(200 ÷ 230) = 41.0° — within 42°, though close to it
- 2R + G: (2 × 200) + 230 = 630 mm — comfortably inside 550 to 700
- Total run: 230 × 12 = 2,760 mm of floor space needed
This passes, but at 41° it is a steep stair. Adding a fourteenth riser drops the rise to 186 mm and the pitch to 39°, which is noticeably easier to climb — at the cost of another 230 mm of floor space.
Common mistakes
- Measuring ceiling height instead of floor-to-floor. The floor structure and the upstairs floor covering both have to be included.
- Forgetting the top riser. Thirteen risers means twelve treads, because the last riser arrives at the upper floor.
- Forgetting the run. A compliant stair still has to fit. Twelve treads at 230 mm needs nearly 2.8 m of floor.
- Uneven steps at the top or bottom. Usually caused by laying flooring after the stair was made. Every rise must be equal.
- Applying private-stair limits to a shared stair. A stair serving more than one dwelling must meet the stricter common-stair limits.
- Overlooking headroom. In loft conversions headroom, not pitch, is usually what fails.
Frequently asked questions
What counts as a private stair?
A stair inside and serving a single dwelling. A stair serving more than one dwelling — for example a common stair in a converted house or a block of flats — is not a private stair and must meet stricter limits on rise and going. Applying private-stair figures to a shared stair is one of the most common failures on flat conversions.
Does replacing my stairs need Building Regulations approval?
Usually yes. Installing a new stair, altering the geometry, or forming a new opening are all building work. A like-for-like repair that changes nothing about the dimensions is a grey area. Ask your local Building Control body before starting — it is far cheaper than discovering a problem at sale.
Why does my stair fail on pitch when the rise and going both pass?
Because pitch depends on both together. A 220 mm rise with a 220 mm going gives 45 degrees, which exceeds the 42 degree maximum even though each individual figure is within limits. Increase the going, add a riser, or both.
What is the 2R+G rule for?
Twice the rise plus the going should fall between 550 and 700 mm. It reflects the geometry of a walking stride: as steps get taller they also need to get shallower for the stair to feel natural. A stair can meet every other limit and still feel awkward if it falls outside this range.
Can I use a steeper stair in a loft conversion?
The 42 degree maximum still applies to a stair serving a habitable room such as a bedroom. Alternating tread stairs are permitted only in tight situations serving a single non-habitable room, and headroom may be reduced to 1,900 mm at the edge in some loft situations. These are narrow exceptions and Building Control must agree them.
Do these figures apply across the whole UK?
Approved Document K applies in England, with an equivalent document in Wales. Scotland works to the Scottish Building Standards technical handbooks and Northern Ireland to its own technical booklets, and the numbers differ. Check the guidance for your nation.
Is what I enter stored?
No. Figures are processed in your browser and never transmitted or retained.
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References
These organisations publish the guidance and figures behind this page.
- GOV.UK — Approved Documents — Approved Document K, which sets the rise, going, pitch and headroom limits used on this page
- LABC — Local Authority Building Control — guidance on when building work needs approval and how to arrange it
- BSI — British Standards Institution — BS 5395-1, the code of practice for the design of straight stairs
Approved Document K applies in England, with an equivalent in Wales. Scotland and Northern Ireland use their own technical standards. Always confirm your design with Building Control before building.