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
Maths
3 formulas
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.
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.
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.
Money
2 formulas
How long before the machine pays for itself. A payback under three years is usually good for a working tool.
Cost 9,000, nets 1,800 a year: 5 years to pay back.
For a flock, a herd, a customer list or a tree that is being improved each year. Shows the real rate behind a rising number.
20 sheep to 50 in 5 years: (2.5^(1/5) - 1) = 20.1% a year.
Convenience only. Double-check the result against the worked example or a second method before you use it.
Agronomy
1 formulas
The way to work out what a crop is worth before you have even got the harvester in the field. Combine it with a per hectare count of plants and ears.
250,000 plants/ha, 1.6 ears, 60 grains, 45 g: (250,000 x 1.6 x 60 x 45) / 1,000,000 = 108 t/ha.
Growing
1 formulas
The halving rule: seed loses half its life each year, so freshness doubles the years it lasts. Buy fresh seed for the crops that go over quickly.
Two-year-old onion seed is about half as good as new.
Workshop
2 formulas
Whether you are rebuilding a gearbox, a mixer or a seedling thinner. Work it out on paper before you order the gears.
A 16 tooth pinion driving a 48 tooth gear is a 3:1 reduction.
Order the bar in lengths that avoid joints in the worst places, and bend a hook at the ends. See Mixing Concrete.
A 12 mm bar lapped 480 mm and a 25 mm cover at each end needs 530 mm of bar per lap.
Construction
1 formulas
The way to sell an idea to whoever holds the purse. Half the cost of an insulation job often comes back in fuel in the first winter.
Insulation saving 900 euro a year for 1,800 euro of work pays back in two years.
Advanced maths
3 formulas
Compound annual growth for a herd, a woodlot, a business or an investment.
100 to 161 in 5 years is 10% a year.
Cooling, spoilage, radioactive decay, drug clearance and declining yields.
1000 kg at 10% loss for 3 years = 729 kg.
Convenience only. Double-check the result against the worked example or a second method before you use it.
Populations, investments, fermentation and anything that grows by a percentage each period.
100 ewes at 8% growth for 5 years = 147.
Convenience only. Double-check the result against the worked example or a second method before you use it.
Finance
7 formulas
A more realistic fall in value for vehicles and electronics that lose value fastest early.
20000 at 20% for 3 years = 10240 book value.
What a sum grows to with compound interest, or what a herd or woodlot will be worth.
1000 at 7% for 10 years = 1967.
Convenience only. Double-check the result against the worked example or a second method before you use it.
Comparing costs or savings across years after inflation. A higher nominal figure may buy less.
1000 in 5 years at 3% inflation is worth 863 today.
The fixed payment that clears a loan over a set number of periods.
10000 at 6% over 5 years is about 193 per month.
Convenience only. Double-check the result against the worked example or a second method before you use it.
How long before a machine, shed or solar array pays for itself. Ignore interest for a quick check.
A 6000 machine saving 1500 a year pays back in 4 years.
What a future payment or saving is worth today. Use it to compare investments.
1967 in 10 years at 7% is worth 1000 today.
Convenience only. Double-check the result against the worked example or a second method before you use it.
Spreading the cost of a machine evenly over its working life.
20000 cost, 4000 salvage, 8 years = 2000 a year.
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.