How we calculate

Every calculator does a real takeoff rather than a square-footage guess. Members are spaced on centre and counted including both ends, coverage is rounded up to whole stock lengths in the sizes lumber and panels are actually sold in, and a waste allowance you control is applied on top. Sheet goods are counted as sheets, concrete as whole bags or quarter-yards of ready-mix, fasteners by the box. The number the tool gives you is a number you can order.

The shared construction helpers all sit in one file so the same rule is applied the same way everywhere — the same on-centre spacing maths, the same rounding, the same bag yield. That matters more than it sounds: it is why two calculators that both need to know how many 80 lb bags fill a cubic yard cannot quietly disagree, and it is the reason a correction fixes every page at once instead of one page at a time.

The span and sizing figures are set at or below the published maximums in the American Wood Council's DCA 6 and IRC Table R507.6, for No. 2 grade sawn lumber under the residential deck load case of 40 psf live plus 10 psf dead. They are deliberately rounded down rather than reproduced exactly, because published spans differ by species and by wet-service condition and because local amendments are common. Rounding down means the tool will never talk you into a longer span than your jurisdiction allows.

Footing depth follows the same logic: you enter your local frost depth and the tool digs past it, because frost depth is the input no national table can supply. Where an input pushes a span or a height past a sensible limit, the result carries an explicit warning rather than silently returning a bigger board. The trade-off is that on a generous species in a mild climate these tools will sometimes specify more lumber than the minimum. That is the direction we would rather be wrong in.

Costs are built line by line: each item in the takeoff carries a per-unit price for the common grade of that material at national-average retail, and the totals are shown as a range rather than a single figure. Nothing is scraped live and nothing is regionalised yet, so treat the range as a budgeting baseline. The breakdown by category is shown next to it deliberately — seeing that lumber is 70 percent of a deck's material cost tells you which input to change.

The estimate covers materials only. It does not include labor, delivery, tool rental, tax, permit fees, disposal or site work, and it assumes you are buying at retail rather than on a contractor account. Prices are reviewed on a rolling basis and every calculator shows the date its figures were last checked, so you can judge for yourself how much weight the number deserves.

MyBuildPlanner is built and maintained by Trace Bailey, who is responsible for the construction maths, the material prices and the guidance on all 91 calculators. Corrections go to hello@mybuildplanner.com and are read by a person. If you find a number that is wrong, that is the most useful thing you can send us — say which calculator and what you expected, and it gets checked against the source.

Drafting is model-assisted. Each calculator is a single configuration file — its inputs, its maths, its guidance and its FAQs — and those drafts are written with AI assistance, then checked against the sources above and reviewed before the page is published. We are telling you this because you should know how anything you are reading was produced. What that assistance never does is decide a span, a footing depth or a price: those come from the published references and are the part a human signs off on.

Every page carries the date its figures were last reviewed. When a price or a rule changes, that date moves and the page is republished; when nothing has changed, the date stays put, which is why it means something.

These are planning tools, not engineered drawings, and nothing here is stamped by a licensed engineer. They are genuinely good at scoping a project, setting a budget, comparing two ways of doing a job and writing a shopping list you can take to a store. They are not a substitute for your local building code, a permit, or a professional's review of anything structural — and they do not know your soil, your seismic zone, your wind exposure or what is buried in your yard.

Confirm footings, spans, setbacks, attachment details and permit requirements with your building department before you buy or build. On anything holding up a roof, retaining more than a few feet of soil, or attaching to your house, get a professional to look at it. The estimate is a starting point for that conversation, not a replacement for it.