Everyday Life Calculations
Cooking, travel, home and time calculations that come up outside of any classroom.
12 calculations published in this category.
Shopping
How To Calculate A Discount
A discount is a percentage taken off the price before tax, so the sale price and the money saved are two separate numbers that the tag never prints. On an original price of 120 with 25 per cent off, the saving is 30.0, the sale price is 90.0, and with 8 per cent tax the bill is 97.2.
sale price = original x (1 - rate); saving = original x rate
How To Calculate Stacked Discounts
Stacked discounts multiply rather than add. A price of 200 cut by 20% and then by 10% falls to 160.0 and then to 144.0, an effective discount of 0.28, not the 0.30 that adding the two rates would suggest.
Final Price = Price x (1 - first) x (1 - second)
How To Calculate Unit Price
Unit price is the price divided by the quantity, expressed per gram, per litre or per 100 g, and it is the only figure that compares two packs of different sizes fairly. The arithmetic is one division; the discipline is applying it before you trust a label.
Unit price = price / quantity
Tipping
How To Calculate A Split Bill
A split bill divides one restaurant bill between several people, and nearly every disagreement about it comes from the order of operations rather than from the arithmetic. The tip is a percentage of the pre-tip bill, it has to be added before anything is divided, and the split only stops being trivial once the shares stop being equal.
tip = bill x rate; total = bill + tip; per person = total / people
How To Calculate A Tip
A tip is just a percentage of a bill, yet most of the arguments around it come from two decisions rather than from arithmetic: what you take the percentage of, and at what point you split it.
Tip = bill x rate; total = bill x (1 + rate)
Home
How To Calculate Electricity Cost
Electricity cost is power multiplied by time, converted into the unit the bill actually uses, then priced. Almost every wrong answer comes from confusing the two quantities — watts and kilowatt-hours — before any arithmetic begins.
Cost = watts x hours / 1000 x rate per kWh
How To Calculate Paint Coverage
Paint coverage is quoted per litre per coat, so the coat count multiplies the area before you divide. The arithmetic is one multiplication and one division; the judgement lies in reading the coverage figure honestly and turning litres into tins you can actually buy.
Litres needed = (wall area x coats) / coverage per litre
How To Calculate Tile Count
Tile count is the surface area divided by the area of one tile, with a wastage allowance added and the answer rounded up. The only real decisions are how much wastage to allow and which direction to round.
Tiles = (room area / tile area) x (1 + wastage)
Travel
How To Calculate Fuel Cost
Fuel cost is two multiplications and one division: distance into litres, litres into money, and money into a rate you can reuse. The only real traps are the units and a consumption figure that runs backwards.
Fuel cost = (distance x consumption / 100) x price per litre
How To Calculate Gas Mileage
Gas mileage is a rate, and it arrives in two scales that run in opposite directions: litres per 100 kilometres, where lower is better, and miles per gallon, where higher is better. Converting between them is a reciprocal, not a single multiplication.
mpg = 100 / (L/100km) x 3.785411784 / 1.609344