Metric and imperial in the estimators

Every estimator that offers metric entry — all but the feet-and-inches and linear-feet tools — takes metres and centimetres, and the construction unit converter moves any figure between systems, but the arithmetic underneath never changes system: the engines compute in feet, inches, square feet, and cubic feet, and a metric entry is converted once on the way in and once on the way out. This guide covers the constants both systems share, the integer "grain" arithmetic that keeps the converter exact, what a tool's unit toggle does, why the 25 kg and 30 kg concrete bags carry manufacturer yields, and how to read a six-significant-digit result.

Everything here is units and quantities. Nothing on this page or in any tool determines structural adequacy, building-code compliance, engineering suitability, or permit requirements — those are questions for a licensed engineer, architect, or code official, as the methodology page sets out.

The definitions both systems share

Converting feet to metres involves no measurement: the imperial units are defined as exact metric quantities, so the factors are exact by definition. The geometry engine holds the length, area, and volume factors and the converter holds the weight ones; the converter derives its grain counts from them, and the unit toggles divide by them:

• 1 ft = 12 in and 1 cu yd = 27 cu ft.
• 1 in = 2.54 cm = 25.4 mm; 1 ft = 0.3048 m; 1 yd = 0.9144 m.
• 1 sq ft = 0.09290304 m² (0.3048 squared); 1 cu ft = 0.028316846592 m³ (0.3048 cubed), so 1 cu yd = 0.764554857984 m³.
• 1 lb = 0.45359237 kg; 1 US short ton = 2,000 lb = 907.18474 kg; 1 tonne = 1,000 kg.

Grains: why the converter is exact

The converter avoids decimal arithmetic until the last step. Within each category it picks a grain so small that every supported unit is a whole number of grains. For length the grain is a tenth of a millimetre: a metre is 10,000 grains, an inch 254, a foot 3,048, a yard 9,144. Area uses that grain squared, volume uses it cubed, and weight uses a hundred-millionth of a kilogram, so a pound is 45,359,237 grains. The full table, from the engine's unit list:

A conversion is (value × grains[from]) ÷ grains[to]: one multiplication and one division. The product is exact whenever it is exactly representable as a double — which every value in the engine header's examples is — and the division is correctly rounded. Those three examples, reproduced through the engine:

• 1 ft → in: 1 × 3,048 ÷ 254 = 12 in.
• 80 lb → kg: 80 × 45,359,237 = 3,628,738,960, ÷ 100,000,000 = 36.2873896 kg — a terminating quotient, so the printed digits are the whole answer.
• 5 cu yd → m³: 5 × 764,554,857,984 = 3,822,774,289,920, ÷ 1,000,000,000,000 = 3.82277428992 m³.

When the quotient does not terminate, no decimal is exact. One metre in feet is 10,000 ÷ 3,048 = 3.28083989501… ft; the engine keeps the nearest double and the converter shows 3.28084 ft. The engine never rounds, its tests check every unit pair there and back, and it accepts values up to 1,000,000,000 in any unit.

Worked conversions

Each row was computed by convertValue at build time: the engine value as far as the display layer keeps it, then the converter's six-significant-digit reading.

The 5 ft 6 in row takes the same path a tool uses for mixed feet and inches — inches ÷ 12 added to the feet, then the grain arithmetic — and lands on 1.6764 m exactly. A tonne is 1.10231 US short tons, so a quote that just says "ton" needs the question asked first.

What the imperial/metric toggle on a tool does

Each estimator with a units toggle has an Imperial/Metric radio pair, and each mode reads only its own boxes: feet (often with an optional inches box) and inches on one side; metres, centimetres, millimetres, square metres, and cubic metres or litres on the other. Whichever set is showing, the field reader converts the entry into the engine's canonical unit before anything else happens: metres ÷ 0.3048 gives feet, centimetres ÷ 2.54 and millimetres ÷ 25.4 give inches, square metres ÷ 0.09290304 gives square feet, and cubic metres ÷ 0.028316846592 gives cubic feet (litres are divided by 1,000 first). The engine then runs exactly as it would for an imperial entry, and the metric readout multiplies the results back out. Three slabs through the concrete slab calculator's engine, all at the 10% concrete waste convention (adjustable in every form):

That last line is the model's honest edge: the quarter-yard step is a ready-mix ordering convention to confirm with the supplier, and the site reports the rounded order in cubic metres rather than inventing a metric increment. The ordering step itself is covered in the cubic feet, cubic yards, and ordering concrete guide.

The factor-of-twelve mistake, and its metric cousin

The classic misreading in concrete estimating is a unit slip in the thickness box. In imperial mode length and width are feet but thickness is inches, so a 4 ft figure typed there as 48 — or a 4 in slab typed as a decimal-foot fraction — is wrong by exactly 12×: the 12 × 10 ft slab at 48 in computes 480 cu ft and a 19.75 cu yd order against the correct 1.75, a factor of 12 in net volume. Metric mode has the same trap with a bigger multiplier: length and width are metres but thickness is centimetres, so 10 cm typed as 0.1 (its value in metres) computes 0.02 m³ instead of 2 m³, a factor of 100. Neither slip is caught by the arithmetic; if a volume looks absurd, read the thickness label first.

Metric concrete bags carry their own yields

The concrete bag calculator switches bag tables with the toggle: imperial compares the 40 lb, 50 lb, 60 lb, 80 lb sizes from the QUIKRETE Concrete Mix (No. 1101) data sheet; metric compares the 25 kg and 30 kg sizes sold in Canada, with yields from Target Products, the licensed manufacturer of QUIKRETE in western Canada. The metric yields are not converted pound figures: mixes are batched and rated per bag by the manufacturer, and the published metric yields sit slightly above the pound rate:

The pound-rate column is 0.0075 cu ft per pound (the 80 lb yield ÷ 80, the rate every imperial size on the data sheet works out to) times the bag's weight in pounds; the tool uses the published figure, converted to litres with the exact cubic-foot factor. The imperial data sheet states its yields as approximate; other brands differ, so check the bag. Every size in both tables is worked through in the concrete bag yields by size guide.

Two metric volumes at the same adjustable 10% waste: 0.5 m³ arrives as 17.66 cu ft, becomes 19.42 cu ft (550 L) with waste, and ceil(adjusted ÷ yield) gives 46 × 25 kg or 38 × 30 kg; 350 L arrives as 12.36 cu ft, becomes 13.6 cu ft (385 L), and rounds up to 32 × 25 kg or 27 × 30 kg. The same 0.5 m³ through the pound table takes 33 × 80 lb bags — a different count because the yields are different products, not because the volume changed.

Six significant digits, not six decimals

The converter shows six significant digits because the sensible number of decimals depends on the units: 36.2874 kg, 0.764555 m³, and 2,204.62 lb are all six-digit readings with the point wherever the magnitude puts it. Values are normalised to strip floating-point artefacts and rounded once for display; the estimator pages round to a few decimals (the examples above use two or four), but the value that feeds a package count is never the rounded one. Package rounding is the other kind, and it always goes up: 2.8775 cu yd became a 3 cu yd order, and 19.42 cu ft became 38 whole 30 kg bags. Display rounding is cosmetic; purchase rounding is a decision.

Where the model stops

Conversions are unit arithmetic on the numbers you enter; they cannot correct a mismeasured dimension or a wrong unit label. The tools do not translate packaging across systems — a 25 kg bag is its own product, not a rescaled 55 lb bag — and they do not know how a metric supplier rounds an order, applies delivery minimums, or varies products by region. And the site-wide boundary applies: quantities only, never structural adequacy, code compliance, or engineering suitability. A perfectly converted quantity is still only a quantity.

Frequently asked questions

Related: the construction unit converter and its printable chart, the concrete bag calculator for both bag tables, the concrete bag yields by size guide, the cubic feet, cubic yards, and ordering concrete guide for the ready-mix step, the feet, inches, and fractions guide for the tape-measure side, and the full list of guides.

The conversions and worked figures on this page are computed at build time by the same engines that run the tools, and dimensions you enter on the tools never leave your browser. The full estimation pipeline is documented on the methodology page.