FORMULA SHELF
12 of 370
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
Maths
12 formulas
Feed price moves, growth rate, water use, milk yield, a price rise at the mart. Always state the direction with a sign.
Feed went from 220 to 260 a tonne: (40/220) x 100 = 18.2% more.
Convenience only. Double-check the result against the worked example or a second method before you use it.
The workhorse of farm arithmetic. Seed rate per hectare scaled to your patch, fertiliser from the label, spray mix from a test plot, feed per animal per day.
If 5 kg of seed covers 2 hectares, 1.5 hectares needs (1.5 x 5) / 2 = 3.75 kg.
Convenience only. Double-check the result against the worked example or a second method before you use it.
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.
On a loan, an investment, or a hire-purchase agreement. The difference from simple interest is that the interest itself earns interest.
1,000 at 6% compounded yearly for 5 years = 1,338.23.
Convenience only. Double-check the result against the worked example or a second method before you use it.
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.
They are the reverse of powers. They are how you find a compound interest time, a half-life, a pH, a decibels level or a Richter magnitude.
log(10)(1000) = 3.
Bracing a gate, checking a diagonal, setting out a rectangle, working the length of a rafter, checking that a frame is square.
A 3 m by 4 m rectangle has a 5 m diagonal.
Convenience only. Double-check the result against the worked example or a second method before you use it.
Mixing feed, splitting a bale, scaling a recipe, dividing a field between two crops, blending a concentrate with a home-grown protein.
A 3:1 mix of concentrate to grain: 12 kg of concentrate needs 4 kg of grain.
How long money takes to double, and how long a debt takes to double as well. Rough, and right to within a few months at normal rates.
At 9% a debt doubles in 8 years.
For very small and very large numbers: soil test results in parts per million, feed analysis in milligrams per kilogram, money in the billions.
0.00042 kg/m3 = 4.2 x 10(-4) kg/m3.
The old hire-purchase and the short-term loan. Easy to check by hand against the paperwork.
500 at 8% for 2 years = 80 of interest.
Convenience only. Double-check the result against the worked example or a second method before you use it.
For a roof pitch, a drain run, a graph of growth against time, or checking whether a field really does slope the way you think.
From (0, 0) to (10, 2) the slope is 0.2, or 1 in 5.
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.