Estimator notes
How this estimator works
Enter a clear opening, baluster width, and your maximum desired gap to calculate a balanced layout with equal clear spaces at both ends. The formula is deterministic, keeps intermediate precision, and rounds only when a material must be purchased in a whole unit.
Typical planning scenarios
- An 8 ft opening with 1 1/2 in balusters and a 4 in maximum gap.
- Several identical openings with extra balusters for cuts or future repairs.
- Metric spacing for a railing project measured in millimetres.
Prices are optional. The quantity result still works when every price field is left empty.
Five solved numerical examples
These expected results were checked from the written arithmetic and are also fixed inputs to the golden-test suite. Comparable calculator checks and domain-expert review remain pending.
1. 2.4 m clear opening
Expected result
- Balusters per opening
- 17
- Total balusters
- 17
- Equal clear gap (mm)
- 95.6
- Centre-to-centre (mm)
- 135.6
Manual derivation
- N = ceil((2400 - 100) / (40 + 100)) = 17.
- Equal gap = (2400 - 17 × 40) / 18 = 95.56 mm; centre spacing = 135.56 mm.
2. Two 1.8 m openings plus spares
Expected result
- Balusters per opening
- 12
- Total balusters
- 26
- Equal clear gap (mm)
- 92.3
- Centre-to-centre (mm)
- 142.3
Manual derivation
- N = ceil(1700 / 150) = 12 per opening.
- Two openings require 24 balusters; adding two spares gives 26.
3. Compact metric opening
Expected result
- Balusters per opening
- 9
- Total balusters
- 28
- Equal clear gap (mm)
- 73
- Centre-to-centre (mm)
- 103
Manual derivation
- N = ceil(920 / 110) = 9.
- The equal gap is (1000 - 270) / 10 = 73 mm; 9 × 3 + 1 = 28 total.
4. Eight-foot opening in normalized millimetres
Expected result
- Balusters per opening
- 17
- Total balusters
- 17
- Equal clear gap (mm)
- 99.5
- Centre-to-centre (mm)
- 137.6
Manual derivation
- 8 ft = 2438.4 mm, 1.5 in = 38.1 mm, and 4 in = 101.6 mm.
- N = ceil(2336.8 / 139.7) = 17; displayed gap is 99.5 mm.
5. Four short openings with repair stock
Expected result
- Balusters per opening
- 7
- Total balusters
- 32
- Equal clear gap (mm)
- 57.5
- Centre-to-centre (mm)
- 77.5
Manual derivation
- N = ceil(540 / 80) = 7 per opening.
- The equal gap is 460 / 8 = 57.5 mm; 7 × 4 + 4 = 32 total.
Common mistakes to avoid
- Measuring to post centres instead of the clear inside opening.
- Treating a common gap preset as a local code approval.
- Rounding an equal gap up after calculation.
Frequently asked questions
Does this confirm a code or permit requirement?
No. This calculator provides a material-planning estimate only. It does not provide structural engineering, permit advice, building-code interpretation, footing-depth approval, installation instructions, or guaranteed quantities.
Can I use metric measurements?
Yes. Switch to metric mode and enter millimetres, centimetres, or metres. The formula uses the same normalized measurements.
Estimate disclaimer
Results are planning estimates. Actual material requirements vary by product, installation method, site conditions, waste, and local rules. Verify product instructions and local requirements. This tool does not determine structural safety, footing depth, permit requirements, installation requirements, or code compliance. Have critical work reviewed by a qualified professional.
Last reviewed: July 16, 2026.