Everyday calculation guides

Calculate a repeatable fuel cost per mile from fill-up records, then keep it separate from a trip estimate.

Measure real-world MPG by dividing the odometer distance between two comparable full tanks by the gallons added at the second fill-up.

MPG and km/L are distance-per-fuel measures, so higher is better; L/100 km is fuel-per-distance, so lower is better. A vehicle using 6.0 L/100 km travels about 16.7 km on one litre, while a vehicle using 8.0 L/100 km travels 12.5 km on one litre.

US MPG and Imperial MPG use different gallons. One US gallon is 3.785412 litres and one Imperial gallon is 4.54609 litres, so 30 US MPG equals about 36.0 Imperial MPG for the same vehicle. A number labelled only “MPG” is incomplete until its gallon convention is known.

An EV charge adds grid energy multiplied by the household electricity rate. Adding 48 kWh to a battery through a 90% efficient charging path requires 53.3 kWh from the meter; at $0.28 per kWh, the energy charge is $14.93. Battery capacity alone is not the billed quantity because charging losses occur before energy reaches the battery.

EV fast charging often slows at a higher state of charge because the battery-management system reduces charging power to control cell voltage and temperature. “80%” is a common planning marker, not a universal switch: one vehicle may taper earlier or later, and the useful question is the charging curve for that battery, temperature, and charger.

A tyre-size change changes wheel circumference, so it can change indicated speed and accumulated distance. Replacing 225/45 R17 tyres with 235/40 R18 tyres increases calculated diameter by about 1.72%; if the original calibration were exact, 100 km/h indicated would correspond to about 101.7 km/h actual. Fitment, legal approval, clearance, and load rating must be checked separately.

Cups measure volume and grams measure mass, so a cup becomes grams only after the ingredient and its density or tested cup weight are named. One US customary cup is 236.588 mL, but 1 cup of flour, sugar, oats, or chopped nuts can have different masses because their particles and packing differ.

When a recipe gives flour in grams, weighing is the most repeatable method. If only cups are available, loosen the flour, spoon it into a standard cup without packing, level the top, and use the recipe’s stated grams-per-cup reference. For a 240 g target at 120 g per cup, measure 2 level cups; do not scoop and compress flour from the bag.

Halving a recipe means multiplying every stated ingredient by 0.5 while keeping the original units and ratios. A recipe with 600 g flour, 420 g water, 12 g salt, and 4 g yeast becomes 300 g flour, 210 g water, 6 g salt, and 2 g yeast. The proportions remain 70% water, 2% salt, and 0.67% yeast relative to flour.

Choose a replacement cake pan by matching both its base area and its usable depth. A 9 by 13 inch pan has 117 square inches of base area; two 9-inch round pans have about 127 square inches. The pair can take a similar quantity of batter, but its depth and baking time still need checking.

Adjust a cake recipe for a different pan by comparing base area, scaling the batter when necessary, and then observing fill level and doneness. A recipe made for a 64-square-inch pan needs about 1.83 times as much batter for a 117-square-inch 9 by 13 inch pan. The number changes ingredient quantity; it does not promise the original baking time.

A coffee-to-water ratio of 1:15 means 1 gram of coffee for every 15 grams of water; 1:16 means 1 gram for every 16 grams of water. For 300 g of water, 1:15 needs 20 g coffee and 1:16 needs 18.75 g coffee. The ratio names mass, not cups, scoops, or the final drink volume.

For four 8-fluid-ounce cups, start with 946 mL, or about 946 g, of brew water and choose a stated coffee ratio. At 1:16, 946 ÷ 16 = 59 g coffee; at 1:15, 946 ÷ 15 = 63 g coffee. “Four cups” is incomplete until the cup volume and coffee-to-water ratio are named.

Dough hydration equals total water divided by total flour, including water and flour already inside a starter. A 100 g starter at 100% hydration contains 50 g water and 50 g flour. With 450 g added flour and 325 g added water, total hydration is 375 ÷ 500 = 75%, not 325 ÷ 450 = 72.2%.

A dough at 60% hydration contains 60 g water per 100 g flour, 70% contains 70 g, and 80% contains 80 g. With 500 g flour, those formulas use 300 g, 350 g, and 400 g water respectively. Higher hydration usually makes dough wetter and potentially more open-crumbed, but flour and technique determine whether it remains workable.

Calendar days count every date on the calendar; business days count only dates defined as working days by the stated schedule and holiday location. Seven calendar days after Monday is the next Monday. Seven Monday-to-Friday business days after Monday reaches the following Wednesday when no public holidays are excluded and the start date is not counted.

Count business days by listing the dates in the interval, applying the relevant workweek, and removing only holidays observed in the location or organisation named by the task. From Thursday 17 April to Thursday 24 April, a Monday-to-Friday calendar has five intervening weekdays after the start; an observed local holiday on Monday 21 April reduces that count to four.

Date-difference answers differ by one day when one method counts elapsed time between date boundaries and another includes one or both named dates. From 1 June to 2 June is 1 elapsed calendar day; counting both 1 June and 2 June as labelled dates gives 2 dates. The correct convention must be stated before arithmetic.

Adding one calendar month means moving to the corresponding date number in the next month when it exists; when it does not, software or a policy must choose a rule. One month after 31 January can be 28 February in 2025, 29 February in 2024, or a different defined result. Adding 31 days is not the same operation.