FORMULA SHELF
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The arithmetic behind the work: what to measure, what to type in, and what the answer comes out as. Every formula says what the symbols mean and gives a worked example, so you can check it against a job you have already done.
SHELVES
Measurement
20 formulas
The workhorse of farm measurement: field areas, yard areas, floor areas and the size of a sheet of plywood. Multiply the two sides, then convert.
A 40 m x 25 m yard = 1,000 m2, which is 0.1 hectare.
Convenience only. Double-check the result against the worked example or a second method before you use it.
Round water tanks, silos, drums, milk churns, water troughs and fuel drums. The single most useful tank calculation there is.
A 1.5 m diameter, 2 m tall tank holds 3,534 litres.
Convenience only. Double-check the result against the worked example or a second method before you use it.
The standard way to compare machines. Fill the tank, work all day, refill and measure. Divide litres by kilometres and multiply by 100.
25 litres over 10 km = 25 L/100km.
Stocking rates, feed weights, seed rates and older books in pounds. Multiply by 2.2046.
500 kg of fertiliser = 1,102 lb.
Convenience only. Double-check the result against the worked example or a second method before you use it.
Oven temperatures, animal temperatures, weather and old American books. Multiplication and addition beat a table every time.
200 C = 392 F, the usual temperature for a hot oven.
Convenience only. Double-check the result against the worked example or a second method before you use it.
Old manuals, old recipes and old spray tables are all in imperial gallons. Divide litres by 4.546.
20 L of fuel = 4.4 imperial gallons.
Old books are in acres, new ones in hectares, and field maps in acres. Learn both. A hectare is about 2.5 acres, which is close enough for a rough job but not for a rent calculation.
5 acres = 2.023 hectares.
Used for round tanks, silage pits, feed bins, pipe ends and round bales. Measure the diameter, halve it for the radius.
A tank of 2 m diameter: r = 1 m, A = 3.14 m2 of base.
Convenience only. Double-check the result against the worked example or a second method before you use it.
Trailer load volume, feed bin capacity, water tank volume, concrete for a footing.
A 3 x 1.5 x 1.2 m trailer = 5.4 m3 = 5,400 litres.
Convenience only. Double-check the result against the worked example or a second method before you use it.
Turn a heap into tonnes. Water is 1 kg per litre. Wheat is about 0.75 kg per litre, fertiliser about 1.1, soil about 1.3-1.5, sand about 1.6, concrete about 2.4.
A 10 m3 heap of wheat at 750 kg/m3 = 7.5 tonnes.
Convenience only. Double-check the result against the worked example or a second method before you use it.
A field with one square end, a yard that narrows, the cross section of a ditch bank, a hipped roof panel.
A field 30 m at the top, 50 m at the bottom, 60 m deep = 2,400 m2.
Convenience only. Double-check the result against the worked example or a second method before you use it.
For wedges, gussets, triangular yard patches, roof end bays and the spread of a spray boom on a slope.
A 10 m base with 4 m of height = 20 m2.
Convenience only. Double-check the result against the worked example or a second method before you use it.
Cutting circles from sheet, sizing a round pen, measuring belt length round a pulley, the girth of a bale.
A 600 mm disc: C = 1,885 mm, so a 1,885 mm belt fits it.
Convenience only. Double-check the result against the worked example or a second method before you use it.
The number that decides whether a job is worth doing. Measure litres per hour by running the machine for an hour with a full tank and a full tank afterwards.
18 L/h at 1.60 per litre = 28.80 per hour.
For spanners, tyres, pipe sizes and imported parts. Divide by 25.4. 12.7 mm is half an inch, 25.4 mm is an inch, 19.05 mm is three quarters.
A 19 mm spanner = 0.75 inch.
Convenience only. Double-check the result against the worked example or a second method before you use it.
Salt in a brine, protein in a feed, moisture in a crop, discount on a sale. Divide the percentage by 100 and multiply.
2.5% salt on 10 kg of meat = 250 g of salt.
Convenience only. Double-check the result against the worked example or a second method before you use it.
Tyre pressures, sprayer pressures, grease guns and air tools. Modern tyres quote bar; old manuals quote psi. 30 psi = 2.07 bar, which is roughly a car tyre.
A sprayer at 3 bar = 43.5 psi.
Convenience only. Double-check the result against the worked example or a second method before you use it.
For land drainage, roof pitch, silage clamp faces, ramps, and whether a load will get up the yard. A 1 in 10 slope is 10%.
1 m rise over 20 m = a gradient of 1 in 20, or 5%.
Convenience only. Double-check the result against the worked example or a second method before you use it.
Work out how fast you are actually travelling. A hectare in a day, a round of the field in an afternoon, or whether a load of sheep will get to the market in time.
12 km in 3 hours = 4 km/h.
Convenience only. Double-check the result against the worked example or a second method before you use it.
Grain heaps, fertiliser heaps, sand and gravel cones, and conical feed piles. A third of the cylinder of the same base and height.
A 3 m radius, 2 m high heap of grain = 18.85 m3 = 18,850 kg at 750 kg/m3.
Convenience only. Double-check the result against the worked example or a second method before you use it.
SOURCES & PROVENANCE
1 listed
This page is an AI-organised index. The label means the grouping and cross-links were compiled with AI; the information itself comes from the public-domain books, reports and manuals listed below. Open any source to read the local copy or follow the link to the original publication.