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
2 formulas
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.
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.
Maths
2 formulas
Average daily milk yield, average egg weight, average tractor hours a week, average weight at weaning.
Weights of 180, 200, 220 and 240 kg give a mean of 210 kg.
For compound growth, interest, scaling up a feed rate, and working out how many times a treatment covers a flock.
2(3) x 2(4) = 2(7) = 128.
Money
1 formulas
Before you buy pigs, take on a shed, or start selling eggs. If the margin per unit is negative, the answer is no, however many units you sell.
Fixed costs 2,400 a year, eggs at 40p with 20p of cost: 2,400 / 0.20 = 12,000 eggs a year, or 33 a day.
Agronomy
6 formulas
The calculation behind a fertiliser plan. Subtract what the soil and the manure already give you, then convert to the product you are actually buying.
Target 120 kg N/ha, soil and manure give 35, using urea: (120 - 35) / 0.46 = 185 kg/ha of urea.
For checking whether there is enough hay for the winter, and for working out how many bales to book in.
5 t/ha of dry matter at 220 kg a bale with 10% loss = 20.5 bales a hectare.
Catch the output from one nozzle for one minute to get flow. Measure your forward speed and the nozzle spacing on the boom. This tells you how many litres per hectare you are putting down, so you can mix the right amount of chemical in the tank.
1.5 L/min x 600 / (8 km/h x 0.5 m) = 225 L/ha.
Convenience only. Double-check the result against the worked example or a second method before you use it.
The rule that makes a heap work. Too much carbon and it sits there for a year; too little nitrogen and it goes sour and smells. See Compost Heap.
A heap of 100 kg straw at 80:1 needs about 2 kg of nitrogen, which is 4.5 kg of urea.
The part of the manure spreadsheet that matters. A load of manure looks like a big pile of nitrogen on the yard, and only a fraction of it is available to the crop this year.
10 t/ha of manure at 0.5% N with 40% availability: (10 x 0.5 x 100 x 0.4) / 100 = 20 kg N/ha.
To work out how long it takes to drill a field, how much fuel you will burn, or how much fertiliser to put on through the drill.
Rows 25 cm apart = 40,000 m of row a hectare.
Livestock
2 formulas
The single number that tells you if a ration or a system is working. Weigh the animals, do it again in 30 days, and divide.
A lamb goes from 30 kg to 42 kg in 60 days = 0.2 kg/day.
For planning how many hives you can keep, how much equipment you need and what the honey is worth. See Keeping Bees.
6 colonies at 25 kg each = 150 kg of honey.
Growing
5 formulas
Bed planning. Work out how many plants you want, divide the bed area by the number, and that is the space each one gets.
Brassicas at 60 cm: 1 / 0.6 x 0.6 = 2.8 plants a square metre, so about 0.4 m2 each.
Work out the total area in square metres, then divide by the area each plant needs (row spacing x in-row spacing). This tells you how many plants to order for a plot or field.
20 x 10 m plot, rows 0.75 m apart, plants 0.3 m apart: 20000 / 0.225 = 88,888 plants.
How much compost to barrow across the plot. One wheelbarrow is about 60 litres, so a 10 m2 bed at 5 cm needs about 5 barrows.
A 2 m x 1.5 m bed at 5 cm: 3 m2 x 0.05 x 1,000 = 150 L, or about 2.5 barrows.
Every bed plan starts with this. A 10 m bed with rows 30 cm apart has 33 rows, and at 25 cm in the row that is about 1,300 plants.
A 10 m bed, 30 cm rows, 25 cm plants: 10 / 0.25 = 40 plants a row.
A cover crop is worth nothing if it is too thin to shade the soil and hold the rain. See Cover Crops.
A vetch and rye mix at 40 + 50 kg/ha gives a canopy in eight weeks.
Workshop
1 formulas
The difference between a saw that cuts along the grain and one that cuts across it. A rip saw has few deep teeth; a crosscut saw has many fine ones.
A 3/32 in gullet gives 1 / (2 x 0.094) = 5.3 TPI, a rip saw.
Energy
1 formulas
How many units of heat you get for each unit of electricity. A COP of 3.5 means the heat pump heats three and a half times more than the resistance heater it replaced.
A 500 W heat pump with a COP of 3.5 gives 1,750 W of heat.
Statistics
2 formulas
How many samples you need before an average is trustworthy.
SD 10 and margin 2 needs about 96 samples at 95%.
How many standard deviations a reading sits from the average. Useful for grading and outliers.
A 70 kg lamb in a 60 kg flock with SD 5 has z = 2.
Finance
1 formulas
How many pens, bales, litres or hours you must sell before covering fixed costs.
Fixed 900, price 12, variable 7: break-even 180 units.
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.