How We Calculate

Every number on this site comes from a formula you can reproduce. This page lists what goes in, what comes out, which assumptions sit underneath, and where the reference data comes from.

The one rule behind everything

Almost every material-sizing mistake in woodworking comes from the same place: nominal lumber sizes. A 1x6 board is actually 3/4 inch by 5-1/2 inches. A 2x4 is 1-1/2 by 3-1/2. Any calculation built on the nominal name overstates coverage by 8 to 9 percent, and over a whole project that difference is dozens of boards and a second trip to the lumber yard.

So every calculator here converts to milled dimensions before it does any arithmetic. Where the comparison is useful, the tool shows both numbers side by side so you can see the gap for yourself.

Rules every calculator follows

• Milled dimensions, not nominal names. Board width and thickness convert to actual sizes from the standard softwood table before use.
• Round up to something you can buy. Sheet counts, box counts, bag counts and board counts always round up with Math.ceil. You cannot order 6.2 sheets of plywood.
• Waste is applied before rounding. A waste factor multiplies the raw quantity first, then the result rounds up, so the allowance is never silently swallowed by the ceiling step.
• Lengths report in inches internally. Feet-and-inches inputs convert to inches at entry, which is why a fraction like 5 1/4 is accepted anywhere a length is asked for.
• Nothing is remembered. No input leaves your browser, nothing is stored, and reloading the page resets everything. There is no analytics on calculator inputs.

Where the numbers come from

The formulas are arithmetic; the reference values are not. These are the sources behind the tables and thresholds:

• Joist and rafter spans — span tables in the International Residential Code, using SPF (spruce-pine-fir) values at 10 psf dead load and 40 psf live load. The IRC is published by the International Code Council.
• Structural design values — the National Design Specification for Wood Construction and its span tables, published by the American Wood Council.
• Stair geometry limits — IRC R311.7: maximum riser 7-3/4 inches, minimum tread 10 inches, with the tighter 7-1/4 inch riser limit above roughly 151 inches of total rise.
• Species density and hardness — the Wood Handbook from the USDA Forest Service Forest Products Laboratory, plus Janka ball hardness values as published for each species.
• Milled lumber dimensions — the voluntary product standard for softwood lumber, the same standard that makes a 2x4 measure 1-1/2 by 3-1/2 inches everywhere in North America.
• Dust collection airflow — the 3,500 to 4,500 feet per minute transport velocity that keeps wood chips suspended in a horizontal duct, rather than the lower heating-and-cooling duct velocities that appear in HVAC references.

Each calculator in detail

The table below is the whole site in one place. Where a formula looks terse, the calculator page itself has a longer explanation under "How this is calculated".

Tool	What it takes in	Core calculation	Main assumption
Board foot	Thickness, width, length, quantity, waste %	(thickness x width x length in inches x qty) / 144, then the waste factor	A board foot is 144 cubic inches; fractions like 8/4 or 5 1/4 are accepted
Lumber waste	Coverage area, board length, nominal size, overlap	Board count from nominal width compared with board count from milled width	Boards are laid edge to edge with the stated overlap only
Deck project bundle	Deck size, board and joist dimensions, spacing, railing length	Decking, joists, beams, posts, footing concrete, screws and railing in one pass	Joists at the stated on-centre spacing; beams sized from the span table
Plywood sheets	Area, sheet size, waste %	Sheets = area / sheet area, times (1 + waste), rounded up	Rectangular layout with no complex cut-outs
Wall studs	Wall length, height, on-centre spacing, openings	Field studs = length / spacing plus one, then corners, ends and 3 per opening	Studs start at a corner; 8 foot studs; three plates
Paint coverage	Room length, width, ceiling, doors, windows, coats, spread rate	(Wall area + ceiling - openings) x coats, divided by the spread rate	Door 21 sq ft, window 15 sq ft, openings not painted
Deck boards	Deck size, board width, board length, gap, joist spacing	Rows from deck width, boards per row from board length, plus joist count	Milled board face width, not nominal; gap counted in the row math
Joist span	Span, joist size, on-centre spacing	Span looked up against the code table for that size and spacing	SPF at 10 psf dead and 40 psf live load
Deck cost	Deck area, board price, railing length, DIY share	Decking plus framing allowance plus railing, then the labour share	Framing budgeted per square foot; railing priced per linear foot
Hardwood flooring	Room size, plank thickness and width, box coverage, waste	Board feet from area and plank section, boxes from coverage with waste	Box coverage as printed on the carton
Fence material	Fence length, height, post spacing, picket width, rail height	Posts = length / spacing rounded up plus one; rails and pickets per row	Milled picket width; about half a bag of concrete per post
Roof pitch	Rise and run	Pitch = rise / run, converted to degrees, X:12 notation and slope multiplier	Straight, uniform slope with no sag
Rafter length	Span, pitch, overhang	Half span and rise give the rafter length by Pythagoras, plus overhang	Ridge at the centre of the span
Stair stringer	Total rise, total run	Risers from rise / 7.5, then identical riser height, tread depth and stringer length	Riser heights must stay within 3/8 inch of each other to pass inspection
Nominal vs actual	Nominal size	Milled dimensions plus the percentage lost in thickness and width	Standard softwood mill sizes
Board feet to square feet	Plank thickness and width	Square feet = board feet / (thickness x width / 12)	Constant plank section
Miter angle	Corner angle, nominal length	Miter = (180 - angle) / 2, bevel = angle / 2, plus the true hypotenuse	Length measured on the inside face of the frame
Dowel spacing	Panel size, dowel diameter, edge distance, rows	Maximum spacing = 2 x diameter, so rows and columns follow from the usable area	Even distribution with the stated edge margin
Screw quantity	Area, joist spacing, board length, waste	Boards from milled face width, two screws per board, plus framing screws at 6 inch centres	Two screws per board is a floor, not a target
Hardwood yield	Log diameter, length	Cylindrical volume x 0.785 for the round-to-square loss, converted to board feet	Straight, sound log; does not account for defect
Cord wood	Stack length, width, height, stove output	Stack volume / 128 for cords, plus piece count, heat and burn time	Stacked, not thrown; seasoned hardwood at about 150 kBTU per cubic foot
Dust collection	Machines in use, duct diameter, run length	Tool CFM plus duct loss, then velocity = CFM x 144 / duct area	Target 3,500 to 4,500 feet per minute in the main run

What these calculators do not do

• They do not replace your building department. Span, stair and guard requirements vary by jurisdiction and by edition of the code. Check locally before you build.
• They do not design connections. Hangers, straps, bolts and their fasteners are engineered items. A material count is not a structural design.
• They do not account for defects. Grading rules, knots, checks and warp all reduce real yield. The waste factor is your margin against that, not a substitute for it.
• They do not see your layout. Cut optimisation, sheet nesting and offcut reuse depend on the actual plan. A quantity is a starting point, not a cutting list.

Corrections

If a number on this site disagrees with your own arithmetic, email [email protected] with the page and the inputs you used. Corrections to a formula are made to the code and reflected here, and the review date at the bottom of every page moves with them.