Methodology

One calculation core, every input mode

Typing dimensions into the form and drawing a room on the grid both feed the same small calculation library. A rectangle's area is always width × length; an L-shape is always a rectangle with a rectangular notch removed, computed with the shoelace formula for polygon area. Nothing about how a shape is entered changes how its area is worked out — that is a deliberate choice, not an accident of how the code is organised, and it's why a room drawn on the grid and the same room typed into the form always agree to the last decimal.

Formulas used

• Rectangle, square, parallelogram: side lengths multiplied directly.
• Circle: π × radius², from a diameter or a radius input.
• Triangle: base × height ÷ 2, or Heron's formula when all three sides are given.
• Trapezoid: the average of the two parallel sides, times the height.
• Sector: π × radius² × (angle ÷ 360).
• L-shape and any drawn polygon: the shoelace formula, which computes the exact area of any simple polygon from its corner coordinates. This is the one routine every other shape's area ultimately reduces to when drawn on the grid.

Every result is computed internally in metric, then converted to whichever unit is displayed — square feet, square metres, square yards or acres — from a single table of conversion factors, so the different units shown for one result can never drift out of sync with each other.

Conversion factors used

All area conversions derive from the international foot, defined as exactly 0.3048 metres, so one square foot is exactly 0.09290304 square metres. The rest follow from that: one square metre is 10.7639 square feet, one square yard is exactly 9 square feet, one acre is exactly 43,560 square feet, and one roofing square is 100 square feet of coverage. The area factors are derived from the length factors in code rather than typed in separately, which is what guarantees a converted figure and a directly computed one always agree.

Gross living area (GLA)

The GLA calculator follows the ANSI Z765 measuring standard used by most US residential appraisers: finished space above grade counts, a finished basement is reported as a separate figure rather than folded into GLA, and openings to the floor below (stairwells, two-storey foyers) are subtracted from the floor above. The calculator applies that standard's arithmetic to the areas you tell it about — it cannot judge whether a space is genuinely finished, heated, or tall enough to qualify, and it is not a substitute for an appraisal.

What is a ballpark figure, not sourced data

Three categories of reference numbers on this site are estimates rather than verified figures:

• Material prices and coverage. The price ranges and per-box/per-sheet coverage figures on the material calculator pages are broad, commonly cited US ballpark figures for material plus typical installation — not current quotes from any supplier. Every material page and the calculator itself say so next to the numbers.
• "What fits in this room" comparisons. The room-size pages count how many beds, cars or tables fit in a room using standard published dimensions (a queen mattress is 60 × 80 in, a US parking space is 9 × 18 ft) laid out in a simple grid with no walkway allowance — a deliberately conservative floor, not a design recommendation.
• "What a room this size is typically used for." Theroom-size pages name the room types a footprint usually suits — a bedroom, a dining room, a one-car garage — by checking its two dimensions against the commonly quoted US residential ranges for each. Those ranges are typical rather than sourced, and they vary by market and by era of construction. The clearance arithmetic printed alongside them (what a 6 ft 8 in bed leaves of a 10 ft width, how many 9 ft bays fit across a 30 ft floor) is exact.

All three are marked as unverified in the source data and are being replaced with sourced figures over time; none of them affects the area math itself, which is exact.