What Is the 27 Rule for Stairs?
Designing a staircase is not simply about connecting one floor to another. A well-designed staircase must be comfortable, safe, and consistent to use. Over centuries of staircase design, builders and architects have developed simple rules to achieve this balance.
One of the most commonly referenced guidelines is known as the “27 Rule for stairs.” While it is not itself a formal law within building regulations, it is widely used by designers and staircase manufacturers as a practical rule of thumb to create stairs that feel natural to walk on.
In this guide we explain what the 27 rule is, why it matters, and how it relates to modern UK building regulations.
Understanding the Basic Stair Components
Before explaining the 27 rule, it helps to understand the two key measurements that define any staircase.
The Rise
The rise is the vertical height between one step and the next.
It is measured from the top surface of one tread to the top surface of the next.
If the rise is too high:
The staircase feels steep
Climbing becomes tiring
The stairs may feel unsafe
If the rise is too low:
The staircase becomes long and inefficient
Walking can feel awkward and unnatural
The Going
The going is the horizontal depth of each step — essentially how far your foot moves forward when stepping up.
If the going is too short:
Your foot may not fit comfortably on the step
Descending the staircase can feel dangerous
If the going is too long:
The staircase takes up unnecessary space
The walking rhythm becomes disrupted
Balancing these two measurements is where the 27 rule becomes useful.
What Is the 27 Rule?
The 27 rule states that:
The rise plus the going of a staircase step should equal approximately 27 inches.
This formula was developed as a guideline to ensure that staircases feel comfortable and natural to walk on.
In simple terms:
Rise + Going ≈ 27 inches
When stairs follow this relationship, the walking motion becomes smooth and intuitive because the human body naturally adapts to that proportion.
Converting the 27 Rule into Metric Measurements
Because the UK uses millimetres rather than inches in construction, the rule is often converted into metric values.
27 inches equals roughly:
685 mm
So in metric terms the rule becomes:
Rise + Going ≈ 685 mm
This is why many staircase designers aim for combinations such as:
| Rise | Going | Total |
|---|---|---|
| 190 mm | 495 mm | ~685 mm |
| 180 mm | 505 mm | ~685 mm |
| 170 mm | 515 mm | ~685 mm |
However, real stair design usually uses slightly different formulas to match modern building regulations.
Why the 27 Rule Works
The reason this rule works is linked to natural human walking movement.
When people walk on level ground, their stride length and body mechanics create a natural rhythm. Staircases interrupt that rhythm because the body must move both vertically and horizontally at the same time.
The 27 rule balances these movements so that:
The body does not need to lift too high
The foot moves forward at a natural distance
The walking rhythm remains consistent
This makes stairs feel comfortable rather than awkward.
Many older staircases that feel particularly pleasant to use unintentionally follow this rule.
How the 27 Rule Relates to UK Building Regulations
In the UK, stair design is governed by Approved Document K of the Building Regulations.
These regulations specify minimum and maximum values for stair geometry.
Typical limits for domestic staircases include:
Maximum rise: 220 mm
Minimum going: 220 mm
Maximum pitch: 42°
While the regulations define safety limits, they do not necessarily guarantee comfort. This is where traditional design rules like the 27 rule can help refine the proportions.
Many staircase designers use both approaches together:
First ensure compliance with Building Regulations
Then optimise the staircase proportions for comfort
The Formula Often Used Instead of the 27 Rule
In modern stair design, a slightly different formula is commonly used:
2R + G ≈ 630 mm
Where:
R = Rise
G = Going
This formula achieves a very similar goal to the 27 rule — ensuring that the stair proportions suit human walking movement.
For example:
Rise = 190 mm
Going = 250 mm
Calculation:
2 × 190 + 250 = 630 mm
This combination produces a comfortable staircase commonly used in UK homes.
Why Loft Conversion Stairs Often Break the Rule
Loft conversions present a unique challenge.
In many existing houses:
Space is limited
Ceiling heights vary
Structural openings restrict staircase size
Because of these constraints, loft stairs sometimes become steeper than ideal.
While they must still comply with Building Regulations, it is not always possible to achieve perfect proportions using traditional rules such as the 27 rule.
Good staircase design in these situations focuses on:
Maximising tread depth
Keeping rises consistent
Maintaining safe headroom
Minimising steepness wherever possible
An experienced staircase designer can often improve comfort even within tight spaces.
Why Consistency Is Just as Important as the Rule
One of the most important aspects of stair safety is consistency.
Every step in a staircase must have:
The same rise
The same going
The same walking rhythm
Even a small variation between steps can cause people to trip because the body expects the same movement every time.
For this reason, professional staircase manufacturers carefully calculate and manufacture each component to precise measurements.
How Professional Staircase Designers Calculate Stairs
When designing a staircase, professionals usually follow a structured process:
Measure floor-to-floor height
Determine the number of rises required
Calculate a comfortable rise height
Design the going to suit the available space
Check compliance with Building Regulations
Refine proportions using comfort formulas such as the 27 rule
Modern staircase manufacturers often use CAD software and specialised stair-design programs to calculate these dimensions accurately.
This ensures the final staircase works both geometrically and ergonomically.
When the 27 Rule Is Most Useful
The 27 rule is particularly helpful in situations such as:
Designing traditional staircases
Planning bespoke timber staircases
Checking whether existing stairs feel too steep
Comparing different staircase layouts
Early stage staircase design
It provides a quick sense check before detailed calculations are carried out.
What Is the 27 Rule for Stairs? – The Takeaway
The 27 rule for stairs is a long-standing guideline used to create staircases that feel comfortable and natural to walk on.
In simple terms:
Rise + Going ≈ 27 inches (about 685 mm)
While modern building regulations set safety limits for stair design, traditional rules like this remain useful for achieving good proportions and a pleasant walking experience.
When combined with professional staircase design and compliance with regulations, these principles help produce stairs that are safe, practical, and enjoyable to use every day.
If you are planning a new staircase or loft conversion, working with an experienced staircase specialist can ensure your design not only meets Building Regulations but also follows the proportions that make stairs feel truly comfortable.
Contact us today to disucss your staircase requirements or use our online staircase builder tool.
