Square footage of a shape that isn't a rectangle
Rooms are rarely rectangles. The reliable method is decomposition: break the plan into rectangles, triangles and circle segments, calculate each and add them. A triangle is ½ × base × height, a circle is π r², and an L-shaped room is two rectangles — measure the two legs separately rather than trying to average a width. For a bay or a curved wall, an ellipse (π × a × b, using the semi-axes) is usually close enough for a materials order. Measure twice and to the actual finished face; a plan dimension taken to the centreline of a stud wall over-states the floor area by the thickness of the wall, every time.
The unit conversions worth memorising
1 m² = 10.764 ft². 1 m³ = 35.31 ft³ = 1.308 yd³. 1 acre = 43,560 ft² = 4,046.86 m². 1 hectare = 10,000 m² = 2.471 acres. For bulk materials the two that do the most work are: one cubic yard covers 100 ft² at 3 in deep, and one cubic metre covers 10 m² at 100 mm. And the one that catches everyone: cubic yards = ft² × inches of depth ÷ 324. That divisor is 12 × 27 — inches to feet, then cubic feet to cubic yards — and it turns a depth in inches straight into the unit a supplier quotes.
Regional land-area units are not universal
A bigha is not a fixed quantity. It is roughly 27,000 ft² in parts of Uttar Pradesh, about 17,400 ft² in Bihar and around 27,225 ft² in Rajasthan, and the katha, biswa and gunta that subdivide it vary the same way. Marla and kanal in Pakistan and northern India have their own regional definitions, and a cent (1/100 acre, 435.6 ft²) and a ground (2,400 ft²) are standard in the south. If you are converting a land record, convert against the state's definition and not against a generic table — the difference between a UP bigha and a Bihar bigha is more than a third of the plot.
Volume, capacity and board feet
A tank's capacity is its internal volume: π r² × h for an upright cylinder, and a rather less friendly circular-segment formula for a horizontal one filled partway — which is why a dipstick reading on a horizontal tank is not linear with volume. One cubic metre is 1,000 litres; one US gallon is 3.785 litres, one imperial gallon is 4.546 litres, and confusing the two is a 20% error. Water weighs 1 kg per litre, so a 1,000 litre tank is a tonne of load on whatever supports it. Board feet, meanwhile, are the lumber trade's own volume unit: thickness × width × length in inches ÷ 144, priced on the nominal size rather than the planed one.
