This professional spindle spacing calculator is designed specifically for UK building regulations. Whether you're installing a new staircase balustrade, landing railing, or exterior deck balusters, this tool will help you calculate the exact number of spindles you need and ensure your layout complies with the UK's 100mm sphere rule for child safety.
Spindles (also called balusters) are the vertical posts that fill the gap between the handrail and base rail of your balustrade. Proper spindle spacing is crucial for both safety and aesthetics, preventing children from slipping through while creating an attractive, evenly-spaced appearance.
How to Use This Spindle Spacing Calculator (UK)
This UK stair spindle spacing calculator is designed to be simple and accurate. Follow these steps:
For Landing or Balcony Balustrades
- Select "Landing / Balcony" from the project type options
- Measure the rail length: Use a tape measure to find the inside distance between your newel posts along the base rail where spindles will be installed
- Enter the spindle width: Measure the thickness or diameter of your chosen balusters
- Choose your mode: Select whether you want to calculate how many spindles you need, or check if your existing spindle count complies
- Click "Calculate Spacing" to see your results instantly
For Stair Flight Balustrades
- Select "Stair Flight" from the project type options
- Measure along the rake: Measure the diagonal length along the angled base rail between newel posts
- Optional - Add pitch angle: For the most accurate UK 100mm rule compliance check, measure your staircase angle (typically 38-42 degrees). This helps calculate the horizontal gap projection
- Enter spindle width and choose your calculation mode
- Calculate to see results tailored for your stair baluster spacing
For Deck or Exterior Railings
- Select "Deck / Exterior" from the project type options
- Measure between posts: Find the distance between your deck railing posts
- Enter your baluster specifications and let the calculator determine the optimal spacing
The calculator will show you multiple layout options including a "Recommended" layout that meets UK building regulations and a "Safer Option" with one additional spindle for extra peace of mind.
How the Spindle Spacing Formula Works
Understanding the mathematics behind baluster spacing calculations helps you appreciate why the results are reliable:
Basic Calculation Method
Our stair balustrade spacing calculator uses this systematic approach:
- Calculate total spindle width: Number of spindles × Width of each spindle
- Calculate total clear space: Total rail length − Total spindle width
- Distribute gaps evenly: Total clear space ÷ (Number of spindles + 1)
This ensures equal gaps between all spindles and at both ends, creating a balanced, professional appearance.
Clear Gap vs Centre-to-Centre Spacing
- Clear gap: The actual open space between spindles (this is what UK building regulations measure for the 100mm sphere rule)
- Centre-to-centre spacing: The distance from the centre of one spindle to the centre of the next (clear gap + spindle width). This measurement is useful when marking out spindle positions
Stair Pitch Angle Adjustment
For angled stair flight balustrades, the horizontal projection of the gap is what matters for the UK 100mm rule. When you enter a stair pitch angle, our calculator:
- Calculates the gap along the angled rail (diagonal measurement)
- Computes the horizontal gap: Diagonal gap × cos(pitch angle)
- Checks the horizontal gap against the 100mm sphere rule for accurate UK compliance
This ensures your staircase spindle spacing meets domestic staircase regulations UK requirements even on steep staircases.
UK Building Regulations for Stair and Balustrade Spindle Spacing
Understanding UK staircase regulations Part K is essential for safe, compliant installations:
The 100mm Sphere Rule (Baby's Head Rule)
The primary regulation for balustrade 100mm gap rule states:
A 100mm sphere must not be able to pass through any opening in the guarding of stairs, landings, or balconies.
This is commonly known as the "baby's head rule 100mm" because it prevents young children from slipping through gaps in your stair railing or balustrade. This rule applies to:
- Internal staircase balustrades in domestic properties
- Landing and balcony railings
- Exterior deck railing balusters
- Any protective guarding where falls could occur
Recommended Best Practice
While the absolute maximum gap between stair spindles UK allows is 100mm, we recommend designing for a slightly smaller gap:
- 99mm or less: This provides a safety margin accounting for measurement tolerances and material variations
- 85-95mm range: Many professional installers target this range for optimal child safety balustrade spacing
- Even spacing: Equal gaps throughout the run create a more professional, aesthetic appearance than irregular spacing
Timber Stair Spindle Spacing vs Wrought Iron
Whether you're installing timber stair spindle spacing or wrought iron stair spindle spacing UK systems, the same 100mm rule applies. However:
- Timber balusters: Typically 32-41mm square or turned profiles
- Wrought iron spindles: Usually 12-16mm diameter, allowing for more spindles in the same space
- Metal balusters: Can be decorative but must still maintain compliant balustrade spacing
Important Disclaimer
This calculator is a guide only. While we've designed this metric baluster spacing calculator to comply with UK building regulations for stair spindles, you remain responsible for:
- Verifying measurements and calculations are accurate
- Checking with your local building control officer or building inspector
- Ensuring your installation meets all applicable building regulations
- Following manufacturer guidelines for your specific spindle products
For critical installations or commercial projects, always consult with a qualified professional or structural engineer.
Common Questions About Spindle Spacing in the UK
What is the maximum gap between stair spindles in the UK?
The maximum gap between stair spindles in the UK is 100mm (approximately 4 inches). This is defined by UK building regulations Part K, which requires that a 100mm diameter sphere cannot pass through any opening in the balustrade guarding. This applies to all domestic and commercial staircases, landings, and balconies. We recommend aiming for 99mm or slightly less to account for measurement tolerances and ensure compliance.
What is the 100mm rule for stair balustrades?
The 100mm rule for stair balustrades (also called the 100mm sphere rule or "baby's head rule") is a UK safety regulation requiring that a 100mm diameter sphere must not be able to pass through any gap in your staircase or landing balustrade. This prevents young children from slipping through the gaps. When calculating stair baluster spacing UK requirements, this is the key measurement to comply with. Our landing balustrade calculator automatically checks your design against this critical safety standard.
How many stair spindles do I need for a 3 metre landing in the UK?
For a 3 metre (3000mm) landing with standard 41mm spindles and a 99mm maximum gap, you would typically need approximately 22-23 spindles. The exact number depends on your spindle width and desired spacing. Use our landing spindle spacing calculator above - simply enter 3000mm for rail length, your spindle width (typically 32-41mm for timber balusters), and click "Calculate Spacing" to get the precise number of spindles required for compliant balustrade spacing.
Does this spindle spacing calculator work for both stairs and landings?
Yes, this stair railing spacing calculator is versatile and works for multiple applications including stair flights (angled balustrades), landings and balconies (horizontal balustrades), and exterior deck railings. Simply select your project type at the top of the calculator. For stair flights, you can optionally enter the stair pitch angle for more accurate horizontal gap calculations, ensuring your staircase spindle spacing meets UK building regulations even on steep stairs.
Can I use this tool for exterior decking balusters?
Absolutely. This deck baluster spacing calculator works perfectly for exterior deck railing balusters. Select "Deck / Exterior" from the project type options, then enter your measurements. The same UK 100mm sphere rule typically applies to exterior deck railings to prevent children from falling through. Whether you're using timber, composite, or metal balusters for your deck, this tool will calculate the optimal even spindle spacing for safety and aesthetics.
Should I design for 99mm or a smaller gap like 90mm?
While the UK regulation allows up to 100mm, we recommend designing for 99mm or slightly smaller for several reasons: it provides a safety margin for measurement variations, accounts for timber movement or manufacturing tolerances, and gives peace of mind that you'll definitely pass building inspection. Many professional installers target 85-95mm for child safety balustrade spacing. If you prefer a closer, more child-friendly spacing, simply adjust the "Maximum Allowed Gap" field in the calculator to your preferred value (e.g., 90mm).
What units does this spindle spacing calculator support?
Our metric baluster spacing calculator supports multiple unit systems for your convenience: millimetres (mm), centimetres (cm), metres (m), inches, and feet. This makes it easy whether you're working with metric measurements common in the UK or imperial measurements from older properties or imported materials. The calculator automatically converts all units to millimetres for accurate calculations and checks compliance against the UK 100mm rule.
How accurate is the stair baluster spacing for steep staircases?
For steep staircases, accuracy improves significantly when you enter the stair pitch angle. Our calculator then computes the horizontal projection of the gap (not just the diagonal measurement along the rake), which is what UK building regulations actually measure for the 100mm sphere rule. Typical domestic staircase pitch angles range from 38-42 degrees. By including this measurement, you ensure your stair pitch and spindle spacing calculations are fully compliant with domestic staircase regulations UK, even on steeper stairs up to 45 degrees.
What's the difference between timber and wrought iron spindle spacing?
The UK building regulations for stair spindles are identical whether you use timber stair spindle spacing or wrought iron stair spindle spacing UK systems - both must comply with the 100mm sphere rule. The main difference is spindle width: timber balusters are typically 32-41mm square or turned, while wrought iron spindles are usually 12-16mm diameter. This means wrought iron installations will generally require more spindles for the same rail length, but create a lighter, more elegant appearance. Both materials work perfectly with this calculator.
Do I need to follow the even spindle spacing formula exactly?
While even spacing creates the most professional and aesthetically pleasing result, slight variations are acceptable as long as no gap exceeds 100mm. Our even spindle spacing formula distributes gaps equally between all spindles and at both ends, which is the industry standard approach. However, if you have architectural features or design preferences that require custom spacing, you can use the "Check spacing for my existing spindle count" mode to verify each section of your balustrade meets UK regulations independently.