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Joist and Rafter Count Calculator

Enter the length of the wall the members run along and the on-center spacing from your drawings. This calculator returns the number of joists or rafters, the blocking and rim board footage, and the board feet to order. It is a counting tool: it will never tell you how far a member can span or what size to use.

ft
Measured perpendicular to the joists or rafters — the direction you lay out the marks on the plate.
ft
The stock length you will buy, including bearing and overhang. For rafters, the length along the slope — not the horizontal run.
count
Extra pieces for trimmers at openings, members under a partition, or a doubled end member. Enter the extra pieces, not the locations.
rows
One block fits between each pair of members, so a row is one fewer block than the member count.
%
Your own allowance. There is no published waste factor for framing members — this number is yours.

Results are estimates for planning purposes. Verify with your supplier, engineer, or a qualified professional before ordering material or performing work.

This calculator does not do spans — and that is deliberate

How far a joist or rafter can carry is decided by species, grade, size, spacing, load and the deflection limit, all together. No calculator that asks you four questions can answer it, and a wrong answer here is a floor that bounces or a roof that sags. Look the span up where it is published, and have it confirmed by whoever approves your drawings:

  • Decks — AWC DCA 6, Prescriptive Residential Wood Deck Construction Guide, free from the American Wood Council, with joist, beam and post tables.
  • Floors, ceilings and roofs — the American Wood Council span publications and the Wood Frame Construction Manual.
  • Engineered members — I-joists, LVL and open-web trusses are sized from the manufacturer's own tables, never from lumber tables.
  • Anything carrying a point load, a hot tub, a wall above or a long overhang — that is an engineer's call, not a table lookup.

Members on layout, and blocking length

Run length12 in o.c.16 in o.c.19.2 in o.c.24 in o.c.
12 ft131087
16 ft1713119
20 ft21161311
24 ft25191613
28 ft29221815
32 ft33252117
40 ft41312621
Blocking cut length10 1/2 in14 1/2 in17 7/10 in22 1/2 in

Counts are for one slope or one floor: floor(run in inches / spacing) + 1. Blocking length is the spacing less the 1 1/2 in dressed thickness of a 2x member (NIST PS 20-20 Table 3). On a gable roof, double every count in this table.

How it's calculated

Members on layout
members per slope = floor( run length (in) / spacing (in) ) + 1
One member on every layout mark, plus the one that closes the end. A 24 ft wall at 16 in on center carries 19.
Total members
total = members per slope × slopes + doubled members, then × (1 + waste %)
A gable roof is laid out twice, once per slope, so the rafter count doubles. A shed roof is one slope.
Blocking pieces
blocks = (members per slope − 1) × rows × slopes
Blocking fills the bays, and there is always one fewer bay than there are members.
Blocking length
block length (in) = spacing (in) − member thickness (1.5 in dressed)
A block between joists laid out 16 in on center is cut 14 1/2 in long. Dressed thickness from NIST PS 20-20 Table 3.
Rim or end board
rim lineal ft = run length × 2 (both ends of the joist run)
Floor and ceiling framing only. On a gable roof the equivalent members are the fascia and the ridge, which are laid out differently and are not counted here.
Lineal and board feet
lineal ft = total members × member length ; BF = nominal thickness × nominal width × lineal ft / 12
Board measure on the nominal size, per NIST PS 20-20 section 2.2.

Frequently asked questions

Careful with the wording: the 24 ft figure in that question is the length of the wall the joists sit on, not how far each joist spans. At 16 in on center, 288 inches divided by 16 is 18 spaces, plus the joist that closes the end, so 19 joists — before trimmers, doubled members and waste.

Because spacing counts the gaps, not the members. Four fence posts make three gaps. The last member lands at the end of the run and has to be added back, and forgetting it is the single most common take-off error on this calculation.

Twice the count for one slope: each side of the ridge is laid out separately. A 30 ft long gable roof at 24 in on center carries 16 rafters per slope and 32 in total, plus whatever is doubled at the gable ends and at openings.

Spacing minus the dressed thickness of one member: 14 1/2 in for joists at 16 in on center, 22 1/2 in at 24 in on center, with 1 1/2 in dressed 2x stock per NIST PS 20-20. Blocking installed in a staggered row is cut to the same length but nailed through the face of the joist.

One fewer than the number of joists, because blocking fills the bays. Nineteen joists make 18 bays and 18 blocks per row.

Five equal spaces in 8 ft — 96 divided by 5 is 19.2. It is the diamond mark on a US tape measure, and it puts exactly five members under every 8 ft sheet of sheathing.

No, and by design. Span depends on species, grade, spacing, load and deflection limit. Look it up in DCA 6 for a deck, in the American Wood Council span publications for a floor or roof, and have it confirmed by whoever stamps or approves your drawings.

Sources & references

Content checked against these sources — last reviewed September 19, 2026.

By — Editorial team of SteelToeTools.com (published by LSEA SAS) Updated v1