How Do I Calculate Staircase Rise, Going, Headroom & Number of Treads?

Summary

Staircase geometry is tightly regulated because poorly designed stairs are one of the most common causes of serious domestic accidents. Getting the rise, going, and headroom right is not just about Building Regulations compliance — it directly affects how comfortable and safe the stair feels to use. Stairs that are too steep or have inconsistent riser heights are dangerous, and Building Control will require correction before sign-off.

The fundamental design task is to divide the total floor-to-floor height into a whole number of equal risers. This sounds simple, but the challenge is fitting the stair into the available plan area within the rise/going constraints, while maintaining adequate headroom. Loft conversion stairs face the most difficulty because they must fit within a restricted space and headroom is often compromised by the roof slope.

Understanding the rules also helps tradespeople identify when an existing stair is non-compliant — important when renovating, replacing balustrades, or advising clients on proposed alterations.

Key Facts

• Maximum rise — private stair — 220mm (Approved Document K, England)
• Minimum going — private stair — 220mm
• Minimum headroom — private stair — 2000mm measured vertically from the pitch line
• Comfort formula — 2R + G should be 550–700mm (R = rise, G = going)
• Pitch angle — private stair — maximum 42° (as a check, not the primary criterion)
• Number of treads — always one fewer than the number of risers
• Minimum handrail height — 900mm above the pitch line (measured vertically from the nosing)
• Baluster spacing — maximum 100mm between balusters (to prevent a 100mm sphere passing through)
• Minimum width — private stair — no minimum in Approved Document K for domestic stairs, but 800mm typical in practice
• Consistency rule — all risers must be equal height and all goings equal depth (max 5mm variation permitted)
• Open risers — allowed on private stairs but balusters must prevent a 100mm sphere passing through
• Tapered treads (winders) — going measured 270mm from the narrow end; must meet the minimum going at this point
• Nosing projection — maximum 25mm overhang of tread nosing beyond riser below
• Scotland and Wales — have slightly different requirements; always check local Building Standards

Quick Reference Table

Going values shown are the Approved Document K minimum (220mm) where the comfort formula G = 650 − 2R gives a result below that minimum; otherwise the comfort formula result is used

Detailed Guidance

Step 1: Measure Total Floor-to-Floor Height

Measure from finished floor level (FFL) of the lower storey to the FFL of the upper storey. This must be the finished floor level — not the structural floor. If the upper floor is timber with 22mm chipboard, add 22mm. If the lower floor is to receive a new tiled finish, add the tile depth. Even a 10mm error in this measurement will cause the last riser to be non-compliant.

Critical tip: Measure in at least two locations and take the larger figure if they differ. Floors are rarely perfectly level.

Step 2: Calculate Number of Risers

Divide total height by a trial riser height and round to the nearest whole number.

Example: Total height = 2,700mm

• Try 13 risers: 2,700 ÷ 13 = 207.7mm → round to 208mm ✓ (under 220mm)
• Try 12 risers: 2,700 ÷ 12 = 225mm ✗ (exceeds 220mm maximum)
• Use 13 risers at 207.7mm

Check with comfort formula: G = 650 − 2 × 208 = 234mm — meets the 220mm minimum ✓

Step 3: Calculate Going and Total Plan Length

The going is the horizontal depth of each tread. Number of treads = number of risers − 1.

Example continued: 13 risers − 1 = 12 treads

• At going of 240mm: total plan length = 12 × 240 = 2,880mm
• At going of 220mm: total plan length = 12 × 220 = 2,640mm

Longer going is more comfortable but requires more plan space. The plan length must fit within the available floor area including any landing at top and bottom.

Landing allowance: Add one going width (220mm minimum) at both top and bottom of the stair for a landing area — Building Regulations require a clear landing at each end.

Step 4: Check Headroom

Headroom is measured vertically from the pitch line (an imaginary line connecting the nosings of all treads) to the nearest obstruction (ceiling, trimmer joist, floor structure above).

Minimum headroom: 2,000mm on the pitch line throughout the full width of the stair.

In loft conversions, the roof slope often cuts through this measurement zone. To check, draw the stair elevation to scale and overlay the roof pitch. The critical point is usually where the roof slope crosses the stair near the top.

If headroom is insufficient: Consider:

• Reducing the going (more compact stair) — but not below 220mm
• Raising the trimmer joist or cutting back the floor structure
• Using an alternating tread device (specialist loft stair) if headroom is critically low and Building Control agrees

Step 5: Verify with the Comfort Formula

2R + G must fall between 550mm and 700mm.

• 2 × 208 + 234 = 650mm ✓ (within 550–700 range)
• 2 × 208 + 220 = 636mm ✓ (also acceptable)
• 2 × 220 + 220 = 660mm ✓ (technically compliant but steep)

Winders (Tapered Treads)

Winders are used where the stair turns a corner without a full landing. Approved Document K requires that the going, measured at 270mm from the narrow end of the winder, must meet the minimum going requirement of 220mm. The narrow end must have a minimum going of 50mm.

Winders are riskier than straight flights and are not preferred in Building Control submissions for new-build properties. They are acceptable in existing buildings where space is very limited.

Balustrade and Handrail Requirements

• Handrails required on both sides if stair is 1,000mm or wider
• Handrail height: 900mm minimum from the pitch line (measured vertically from nosing)
• Landing handrail height: 900mm from landing level
• Balusters: maximum 100mm clear gap (sphere test — a 100mm ball must not be able to pass)
• Open riser stairs: balusters must prevent a 100mm sphere passing through the open riser
• Glass balustrading: must be toughened or laminated safety glass per Approved Document K and BS 6180

Frequently Asked Questions

My existing stair has a rise of 225mm — is it illegal?

An existing stair that predates current regulations is not automatically illegal. Building Regulations apply at the time of construction, and older stairs were built to older standards. You only need to comply with current Approved Document K if you are replacing the stair or carrying out work that triggers Building Control notification. However, it is good practice to inform clients of any safety deficiencies.

Can I have an open riser on a new stair?

Yes, Approved Document K permits open risers on private stairs, provided that a 100mm sphere cannot pass through the opening between treads. In practice, this means balusters or other infill must be installed between treads if the gap would otherwise exceed 100mm. Open risers are not permitted on stairs used by young children (e.g. access to an area specifically designed for children).

How many winders can I use in a quarter-landing?

There is no fixed maximum number of winders, but Approved Document K requires each to meet the going requirement at 270mm from the narrow end. In practice, three winders in a 90° turn is typical. Using more winders creates narrower individual treads and increases the risk of slipping. A full quarter-landing (full-width landing at the turn point) is always the safer design.

What's the minimum width for a domestic stair?

Approved Document K does not specify a minimum width for a private domestic stair (as of the current edition). However, NHBC standards require a minimum of 800mm clear width for new-build dwellings. Fire escape requirements (Part B) may require 750mm minimum clear width on escape stairs in purpose-built flats. Check with Building Control for the applicable requirement on your specific project.

What is an alternating tread stair?

An alternating tread device (sometimes called a space-saver or ship's stair) has steps that alternate left-right, allowing a steeper pitch in restricted headroom situations. They are only permitted where a conventional stair would be impractical, specifically for access to a single room in a loft conversion per Approved Document K. They cannot be used as a main stair in a dwelling. They still require a handrail each side and minimum headroom of 1,800mm.

Regulations & Standards

• Approved Document K — Protection from falling, collision and impact; England and Wales; stair design requirements

• BS 5395-1 — Stairs; code of practice for the design of straight stairs

• BS 6180 — Barriers in and around buildings; balustrade and guard requirements

• Building (Scotland) Regulations — Technical Handbook Section 4 — Scottish equivalent of Approved Document K

• Approved Document B — Fire safety; width requirements for escape stairs in flats and HMOs

• HM Government — Approved Document K — Official staircase regulations

• NHBC Standards Chapter 8.1 — Stair design standards for new-build properties

• BSI BS 5395-1 — Detailed code of practice for straight stair design

• Staircase Trade Association — Practical guidance and worked examples

• staircase regs — Full staircase regulations including balustrade, fire escape, and handrail detail

• loft conversions — Loft conversion regulations including stair headroom requirements

• building control — When to notify Building Control for staircase work

• escape routes — Means of escape requirements affecting stair design