Calculators
Joist Calculator
Estimate floor or deck joists to buy from run length, on-center spacing, and waste allowance locally in your browser.
Enter the joist run length and the on-center joist spacing in feet to estimate how many floor or deck joists to buy, including a waste allowance for cuts, culls, and blocking. The calculation runs locally in your browser; your values are not uploaded or stored.
Enter the spacing in the same unit as the run; for example, 16 inch on-center spacing is about 1.33 ft and 24 inch spacing is about 2 ft. This estimate covers one straight joist run and counts the end joist, but ignores rim joists, beams, hangers, blocking, and openings, so measure each run separately and add extra waste for openings. Check your lumber supplier because joist lengths and grades vary by product. This joist estimate is a planning guide, not a purchase guarantee.
Find how many joists to buy without uploading anything.
Enter the joist run length plus the on-center joist spacing and a waste allowance to see the base joist count with the end joist and the rounded-up buy count. Everything runs locally in your browser, so your values are not uploaded or stored.
Frequently Asked Questions
Everything you need to know about this tool, how it works, and privacy.
What does the joist calculator show?
Enter the joist run length, the on-center joist spacing in the same unit, and a waste allowance. The calculator returns the base joist count including the end joist plus the buy count with waste rounded up, with the calculation shown. For example, a 20 ft run with 1.33 ft (16 inch) spacing and 10% waste needs 16 base joists, so buy 18 joists.
Which formula does the joist calculator use?
It uses base joists = floor(run length / joist spacing) + 1 for the end joist, then joists to buy = base joists x (1 + waste percent / 100) rounded up. Use the same length unit for the run and the spacing; for example, 16 inch on-center spacing is about 1.33 ft and 24 inch spacing is about 2 ft. The estimate covers one straight joist run and ignores rim joists, beams, hangers, blocking, and openings.
Which inputs are accepted?
Enter a joist run length and joist spacing greater than zero through 1000000000, and a waste allowance from 0 to 100 percent (blank means 10). Decimal lengths and spacing values are accepted. Zero, negative numbers, non-numeric text, and out-of-range values show a clear error instead of a result.
Does the joist calculator upload my values?
No. The whole calculation runs locally in your browser. Your run length, spacing, and waste allowance are not uploaded, stored, or sent to a server.