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Deck joists Calculator

Estimate joists from a specified centre spacing.

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How to use this calculator

Your measurements

Choose the unit beside each measurement. Changing a unit converts your value.

Item name (optional)

View your estimate

The spacing must come from an appropriate design. The calculator counts pieces and does not check load capacity.

YOUR ESTIMATE

Joists to buy

Count before waste15

Total material length40.8 m

Whole quantities are rounded up.

PLAN YOUR PROJECT COST

Pricing calculator

Use your supplier’s rate or enter a quoted total. Save each estimate to your project and print everything together.

What are you pricing?

Price is per

Price (optional)

Currency label

Your selected quantity already includes any allowance shown in that result. Check whether you are pricing individual items or whole packs. Delivery, tax and labour are not added automatically. A quoted total applies only to this saved calculation.

Deck joists Calculator: measuring notes and method

Count across the joists

Enter the distance across the parallel members as Run length and each member’s length separately. The model includes members at both ends of that run before adding extras.

Count additional members

Rim members, blocking, doubled joists and opening supports need explicit quantities alongside the parallel-member count.

Formula

Pieces before waste = round up(run ÷ maximum spacing) + 1 + extras. Apply waste and round up.

Worked examples in metric and imperial units

Illustrative values match Reset example and stay fixed when you edit the calculator. Both columns describe the same calculation; converted inputs are rounded.

Metric worked example

Run length: 6 m
Maximum centres: 450 mm
Length per piece: 2.4 m
Extra pieces for ends / openings: 0
Waste allowance: 10 %

Joists to buy: 17

Imperial worked example

Run length: 19.685 ft
Maximum centres: 17.7165 in
Length per piece: 7.874 ft
Extra pieces for ends / openings: 0
Waste allowance: 10 %

Joists to buy: 17

Follow the calculation

Lengths in this working are in metres.

Intervals = 6 ÷ 0.45, rounded up.
Add one end piece and 0 extra pieces, then multiply by 1.1 and round up.
Each counted piece is 2.4 m long.

Reviewed 19 October 2026. How calculations are checked · Report a correction

Related calculators

Decking CalculatorEstimate full-length board rows across a deck.Deck fixings CalculatorEstimate screws from board-to-joist intersections.Deck coating CalculatorEstimate deck oil, stain or coating from measured surface area and product coverage.

PRACTICAL READING

Understand the inputs. Check the order.

Measurement methods and worked examples to use alongside this calculator.

Count deck boards across the width and cuts along the lengthBoard width, gaps and stock lengths create two separate quantity checks.
Estimate deck fixings and finish from the selected layoutCount fixing locations and coated faces separately from the deck footprint.
Build a measurement register you can check laterRecord where each number came from before it becomes part of a material order.
Choose an allowance you can explainSeparate measured quantity, cutting loss and uncertainty instead of adding an automatic percentage everywhere.

Browse all guides →