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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.

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.

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 cover crop is worth nothing if it is too thin to shade the soil and hold the rain. See Cover Crops.

How many standard deviations a reading sits from the average. Useful for grading and outliers.

How many pens, bales, litres or hours you must sell before covering fixed costs.

How many samples you need before an average is trustworthy.

Old manuals, old recipes and old spray tables are all in imperial gallons. Divide litres by 4.546.

Average daily milk yield, average egg weight, average tractor hours a week, average weight at weaning.

For planning how many hives you can keep, how much equipment you need and what the honey is worth. See Keeping Bees.

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.

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.

For compound growth, interest, scaling up a feed rate, and working out how many times a treatment covers a flock.

Bed planning. Work out how many plants you want, divide the bed area by the number, and that is the space each one gets.

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.

For checking whether there is enough hay for the winter, and for working out how many bales to book in.

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.

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.

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.

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.

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.

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.