The footprint and the framing go in; the lumber order comes out — joists, rim, ledger, beam and posts checked against the IRC span tables, then deck boards, screws or hidden clips for the surface.
What Your Results Mean
The framing panel is the structure order. Joists come from spacing arithmetic — deck length divided by on-center spacing, rounded up, plus one — and each joist runs the full deck width, which is why the span verdict sits directly under the count: the width you typed is checked against what that joist at that spacing is allowed to carry, and if it fails, the sub-line names the fix. Perimeter members close the frame (ledger and rim along the length, end joists along the sides), the beam comes as two plies with splices staggered, and the post count comes from the beam table's maximum spacing — the calculator distributes posts evenly and shows the actual spacing it used, which will always be at or under the table's cap.
The decking panel is the surface order. Coverage is the board's true face plus the gap — the number the label never says out loud — and lineal feet is simply area converted through it. Boards divide the footage by your stock length with waste on top, and the fastener row prices the two schemes honestly: face screws at two per board-joist crossing, hidden clips at one. Switch fastening and the gap field follows (clips fill in Trex's 3/16 in), because the fastener, not the installer, sets composite side gaps.
Reading the Lines
- Span verdict: OK with headroom is comfort; at the limit means the table allows it with nothing to spare; over span means upsizing, tightening spacing, or a mid-span beam — the sub-line prices the first two.
- Joists: count × span. The last space may run short when the length doesn't divide evenly — the rim closes the remainder.
- Rim, ledger & ends: lineal feet and the boards they cut from, waste included. A splice in the ledger lands on blocking, not mid-air.
- Posts & footings: post spacing is center-to-center; each post wants a footing below frost depth — take the count to the concrete calculator for the bag or yard figure.
- Rows: boards across the width; the final row is usually ripped to width, which is part of what the waste allowance absorbs.
- Fasteners: two screws per crossing or one clip per crossing. Composite at 24-in spacing draws a warning — the board's own rating caps first.
What the Calculator Does Not Do
It sizes the flat frame and the surface — not the project. Stairs and railings are separate orders: stringers follow your rise and run, rails follow perimeter and post layout. Hardware beyond hangers — hurricane ties, post bases, ledger lags, the beam's nailing schedule — is sold by the box with code-specified counts the lumberyard knows by heart. It doesn't price lumber (boards per lineal foot move weekly and by region), and it doesn't inspect: footing depth, flashing behind the ledger, and guard post connections belong to your building department. What it does guarantee is that the arithmetic under your order is shown, sourced, and repeatable.
Pro Tip
Order the whole surface from one lot: mill runs color-shift, and the three boards you buy next week will tattoo a shade line across your deck. The deck lumber guide walks every formula here by hand, waste and stagger included.
The Math Behind the Calculator
Framing is counting; decking is one conversion. The span tables doing the structural work are printed below, so the verdicts above are checkable to the source:
joists = ceil(length × 12 ÷ spacing) + 1
boards = ceil(lineal_ft × (1 + waste) ÷ stock_length)
posts = beams × (ceil(length ÷ max_post_spacing) + 1)
deck_LF = area × 12 ÷ (face_width + gap)
screws = 2 × rows × joists · clips = rows × joists
Joist capacity and beam capacity come from the IRC's prescriptive tables, Southern Pine No. 2 at the code's 40 psf live + 10 psf dead load — the same columns your inspector reads:
Maximum joist spans, IRC Table R507.6. Cantilevers past a beam cap at one quarter of the actual back-span.
Maximum post spacing, IRC Table R507.5, read by the joist span the beam carries. Joist spans past 12 ft use the last column; between columns the calculator interpolates.
The coverage arithmetic works in any unit because it is a ratio: face plus gap over the area's width direction. Metric mode converts lengths, areas and spacings throughout — board, package and screw counts stay piece counts, because that is how they sell.
Assumptions & Limitations
Every number above rests on these constants. If your deck or product differs, change the matching input — or read the linked guide for the full reasoning.
- Span tables: IRC R507.6 (joists) and R507.5 (2-ply beams), Southern Pine No. 2, 40 psf live + 10 psf dead — Douglas Fir-Larch and Hem-Fir run shorter; read the grade stamp
- Joists counted at both ends: ceil(length ÷ spacing) + 1, last space allowed to run short; each joist runs the full deck width
- Beam table read by full joist span, capped at the 12-ft column, linear between columns; beams assumed 2-ply with splices staggered
- Perimeter: ledger + rim + 2 end joists attached; freestanding swaps the ledger for a second rim and second beam, doubling posts
- Board faces: 6-in nominal = 5.5 in actual, 4-in nominal = 3.5 in — both editable; gaps default 1/4 in wood, 3/16 in clip-set (Trex standard)
- Waste allowance applies once to all lumber at the percentage you set — 10% default, 5% for dead-straight rectangles, 15% for diagonals
- Fasteners: 2 screws or 1 clip per board-joist crossing; hidden-clip counts exclude starter/finish screws and picture-frame breaker hardware
- Stairs, railings, guards, hardware schedules, footing concrete and flashing are outside this takeoff — the hanger count is the reminder, not the hardware list
- Metric mode converts lengths, areas and spacing; board and fastener counts are piece counts everywhere
Worked Example: The 20 × 12 Attached Deck
The same deck walked through in the deck lumber guide, so every number below is checkable against it. Leave the defaults as they load: 20 × 12 ft, ledger-attached, 2×10 joists at 16 in on center, 2-ply 2×10 beam, 12-ft framing stock, 5.5-in boards face-screwed with a 1/4-in gap, 16-ft decking, 10% waste:
- Joists: 240 ÷ 16 = 15 spaces + 1 = 16 joists, each 12 ft — 192 LF → 18 boards at 12-ft stock.
- Span verdict: a 2×10 at 16 in spans 14 ft 0 in — OK, 2 ft of headroom.
- Perimeter: 20-ft ledger + 20-ft rim + two 12-ft ends — 64 LF → 7 boards.
- Beam: 2-ply × 20 ft = 40 LF → 4 boards; the 12-ft joist span caps posts at 6 ft 11 in, so 20 ft needs 3 spaces — 4 posts at 6 ft 8 in, 4 footings, 16 hangers.
- Decking: 240 sq ft × 12 ÷ 5.75 = 501 LF, 26 rows → 35 boards at 16 ft; fasteners: 2 × 26 × 16 = 832 screws.
Now stress it. Set the width to 14 ft with the joists still at 2×8 × 16 in: the verdict flips — over span by 2 ft 2 in, and the sub-line shows the way out, a 2×10 reaching exactly 14 ft 0 in with zero margin or a mid-span beam splitting the run into two 7-footers. Tighten the 2×8s to 12 in and you get to 13 ft 1 in — still short, which is exactly the case where the third fix (the beam) is the only honest one.
Switch fastening to hidden clips: the gap field fills 3/16 in, coverage tightens to 5.6875 in, and the fastener line drops to 416 clips — with Trex's 90-count box covering 50 sq ft at 16-in spacing, that's five boxes for this deck. And select freestanding: the second beam doubles the order to 8 beam boards and 8 posts, the rim doubles to 4 boards, and the hanger count zeroes out because nothing hangs from the house anymore. Every one of those numbers is the formula above, not a guess behind a button.
Run your own deck before believing any of it — tape the footprint off the house with string, decide which dimension the joists run, and let the verdict argue with your plan before the lumberyard does.