Deck calculators that show the math

Most deck calculators give you a number and stop. This one gives you the number, the formula behind it, the inputs it used, and a plain statement of what it does not know. You should be able to check every result by hand.

Deck calculators

Deck board calculatorRows across the deck, linear feet, waste, board counts by stock length, fasteners. Deck stair calculatorRiser count and height, tread depth, total run, stringer length, IRC limits. Deck railing calculatorPosts, baluster count, actual clear opening checked against the 4″ sphere rule. Deck joist calculatorJoist count, real spacing, total linear feet, rim joists, blocking rows. Footing concrete calculatorVolume per hole, total cubic yards, 60 lb and 80 lb bag counts. Deck cost estimatorMaterial and labor ranges by decking type and build height. Board foot calculatorLumber volume and cost for hardwood priced by the board foot.

Lumber and materials

Lumber weight calculatorWeight per piece and per order by species and moisture content. Board feet to linear feetThe conversion, which only exists once you fix the cross-section. Plywood sheet calculatorSheet count for an area, with waste and layer multiplier. Paint and stain coverageGallons by coating type, coats and how porous the wood is.

Framing and structure

Post hole calculatorConcrete and gravel per hole, with the post volume subtracted. Ledger attachment guideFastener schedules, the flashing sequence, and where a ledger is not allowed. Reading a span tableWhy we will not publish a span, and how to read the table that has one. Cantilever rulesThe quarter-back-span limit and what it does to a layout.

Outdoor living

Pergola rafter calculatorRafter count, length with overhang, and the shade percentage the spacing produces. Patio paver calculatorPavers, loose base volume, sand and edge restraint. Fence calculatorPosts, rails, pickets and concrete per hole. Privacy screen calculatorSlats, linear feet and the actual closed proportion. Deck lighting calculatorLoad, current and voltage drop, with the maximum run per gauge.

Why this site exists

Search for a deck calculator and you will find dozens that will happily tell you that a 12 × 16 ft deck needs 29 boards. Almost none will tell you which board width they assumed, whether they used nominal or actual dimensions, what gap they applied, or how much waste they built in. Change any one of those and the answer moves by 10% — more than the difference between a successful order and a second trip to the yard.

So every tool here does three things: it prints the formula, it shows the substitution with your numbers, and it says which inputs it is not modelling.

What we do not do

We do not size structural members. Joist and beam depth depends on species, grade, spacing, live load and deflection limit. That requires a span table and, often, an engineer. Our joist tool counts and lays out — it does not approve a span.
We do not interpret your local code. Where we quote a dimension (railing height, riser maximum, sphere rule) it is from the IRC as published, and we say so. Your jurisdiction may have amendments.
We do not guess at site conditions. Soil bearing, frost depth and slope are inputs we cannot see. Where they matter, the tool says so instead of silently assuming an average.

How the numbers are produced

Every calculator is a deterministic function of the values you type — no server, no stored profile, no model. The arithmetic runs in your browser. Worked examples on each page are computed with the same function the tool uses, so you can reproduce them with a pencil. If a formula changes, the page is re-dated. See methodology for the full account.

Questions people actually ask

Are these calculators free?

Yes, all of them, with no account and no limit. The site is funded by ad placement, which is disclosed on the affiliate disclosure page. No advertiser has any input into a formula or a result.

Should I use nominal or actual board width?

Actual. A nominal 5/4×6 deck board measures 5.5 inches across the face, not 6. Using 6 overstates coverage by about 9% and you will come up short. Every tool on this site asks for face width for that reason.

Why does my board count differ from the yard's?

Usually one of three things: they used nominal width, they applied a different gap (1/8″ vs 1/4″), or they rounded up to whole boards per row rather than across the deck. Our board tool prints the coverage width it used so you can compare directly.

Do you store what I type?

No. The calculation runs entirely in your browser and nothing is transmitted. There is no account, no save, and no server-side processing of your measurements.

Deck board calculatorRows, linear feet, waste and fastener counts. Deck stair calculatorRisers, treads and stringer length with IRC limits. Deck code checklistThe dimensions that actually get inspected.

Deck calculators that show the math