Joist Calculator: 18 Joists for a 20 ft Floor, 16 in Spacing

What does joist needed 18 joists mean?

It means eighteen joists frame a 20 ft floor once cutting waste is allowed for. The floor itself is 20 feet long; at one joist every 16 inches the bare count is 15 bays plus the end joist, or 16 joists, and the 10% waste allowance adds 2 joists for the joists cut at the ends and the culls rejected for bow or twist, taking the order to 18 joists, or 216 feet of joist in total. The waste joists are not spare stock exactly — they are the joists the floor consumes in cutting and culling, so a floor ordered without them comes up short at the last bay.

Output · formula · meaning

Three things to check before you trust the number

First, check the spacing figure against the plans: 16 in is a planning figure, but a floor carrying a heavy load can call for 12 in, and at 12 in this floor needs 24 joists, not 18 — spacing moves the whole order. Second, measure the floor the joists span, not the room around it: the count follows the floor length, and an opening the joists skip should come off the length first. Third, judge the waste honestly: a straight run fits 10%, but a floor with many openings can want 15%, since every opening means a cut joist and a trimmer.

Worked example

20 ft long, 16 in spacing, 12 ft joists, 10% waste. Joists exact: 20 × 12 ÷ 16 + 1 = 16. With waste: 16 × 1.10 = 17.6, rounded up to 18 joists. Total length: 18 × 12 = 216 ft. In practice that means ordering 18 joists of 12 ft, and expecting the last joists to go on the end cuts and the culls. If the same floor used 24 in spacing instead, the need would fall to 13 joists with waste, but the floor framed would be identical.

Every branch, checked

With a 24 ft long floor instead of 20 ft, the joist count rises to 21 joists with waste, or 252 ft total. With 24 in spacing, the count falls to 13 joists with waste, or 156 ft total. With 16 ft joists, the count stays 18 joists but the total rises to 288 ft. With 0% waste, the count falls to 16 joists, or 192 ft total. Floor length, spacing, joist length and waste each move the order on their own, so a wrong spacing is as costly as a wrong measurement.

The reverses, worked backwards

The length behind 18 joists is 20 ft: at 16 ft the need is 15 joists (180 ft total, 16 ft floor, 16 in spacing, 12 ft joists, 10% waste); at 24 ft it is 21 joists (252 ft total, 24 ft floor, 16 in spacing, 12 ft joists, 10% waste). The spacing behind 18 joists is 16 in: at 12 in the need is 24 joists (288 ft total, 20 ft floor, 12 in spacing, 12 ft joists, 10% waste); at 24 in it is 13 joists (156 ft total, 20 ft floor, 24 in spacing, 12 ft joists, 10% waste). The waste behind 18 joists is 10%: at 0% the need is 16 joists (192 ft total, 20 ft floor, 16 in spacing, 12 ft joists, 0% waste); at 20% it is 20 joists (240 ft total, 20 ft floor, 16 in spacing, 12 ft joists, 20% waste). Each reverse moves one input and holds the rest, so the step is exactly one dimension or one waste change.

What the defaults assume

The defaults assume a 20 ft straight floor, joists every 16 in, 12 ft joists, and 10% cutting and culling waste. They do not assume a second floor, blocking between joists, rim joists or hangers — a floor with a stair opening needs its trimmers and headers counted separately, and the blocking is bought by the length, not by the joist.

Who uses this

• A homeowner framing a new floor: the 18-joist order settles the lumber call before the first joist is cut.
• A DIY builder decking a new room: the same maths on the floor alone gives the joists, not a whole house.
• A contractor quoting a job: floor length, spacing, waste — the quote follows the same three steps, and the waste line shows the client why.
• A landlord budgeting a floor repair: the floor size off the plans, divided by spacing, gives the joist line on the repair budget.

Accuracy & limitations

The arithmetic is exact for the inputs entered; the judgement is the spacing figure and the waste allowance. Real floors have stair openings, corners and headers, and the spacing on the framing plan overrides any planning figure; a floor measured gross over-orders, and a spacing read off a different drawing under-orders. Treat 18 joists as the planning order for a straight 20 ft floor at 16 in spacing, and raise the waste to 15% for a floor with many openings.

Cite this calculator

Bright Side Kit, “Joist Calculator: 18 Joists for a 20 ft Floor, 16 in Spacing”, https://brightoolbox.com/joist-calculator/, accessed 3 November 2026. Students and researchers: cite the calculator name, the page URL and the date you used it, and quote the defaults (20 ft floor, 16 in spacing, 12 ft joists, 10% waste) so the 18-joist result can be checked.

Frequently asked questions