What load can my floor joists support?

Load capacity depends on joist size, spacing and span. A 47×200mm joist at 400mm centres spanning 4m can safely carry approximately 1.5–2.0 kN/m² for domestic use. Increasing joist depth or reducing spacing increases capacity.

What is the imposed load for a domestic floor?

UK Building Regulations require domestic floors to support a minimum imposed load of 1.5 kN/m² (approximately 150 kg/m²). This covers normal residential use including furniture and people.

What is a point load vs distributed load?

A distributed load is spread evenly over the floor area (like furniture and people). A point load is concentrated at a single point (like a heavy appliance, piano or safe). Joists can carry a higher total distributed load than point load.

Dead load is the permanent weight of the floor structure itself: joists, boarding, plaster ceiling below, flooring finish. Typical dead load is 0.4–0.6 kN/m² for a standard domestic floor.

When should I get a structural engineer involved?

If your planned loading exceeds the joist capacity, if you’re removing walls, adding a bathroom (heavy point loads from bath), or converting a loft. Always consult a structural engineer for commercial or unusual loading.

TradeCalculator

Floor Load Capacity Calculator — Joist Size & Safe Loads

Check joist size and spacing against safe distributed and point loads, then compare against your planned use.

Joist Size

Cross-section dimensions

Joist Spacing

Joist Span

Clear span between supports

Planned Use

Determines imposed load

How We Calculate This

This calculator determines the safe distributed and point load for your floor joists, then checks whether your planned use is within capacity.

The formula

Max UDL = 8 × Moment capacity ÷ (Span\u00b2 × Spacing × Safety factor)

Max point load = 4 × Moment capacity ÷ (Span × Safety factor)

Utilisation = Applied moment ÷ Joist capacity × 100%

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.