Everyday calculation guides — page 4 of 4

Combine each semester’s credit-weighted grade points into one cumulative total instead of averaging semester GPAs.

Convert a height in feet and inches to centimetres with the exact 25.4 mm-per-inch factor, and see why the reverse conversion rarely lands on a whole inch.

Convert between mechanical horsepower, metric horsepower (PS), and kilowatts with their three separate exact-ish defining constants.

Convert a weight given in stone and pounds to kilograms with the exact 6.35029318 kg-per-stone factor used across the UK and Ireland.

Convert area between square feet, square metres, acres, and hectares using the exact 0.09290304 sq m-per-sq ft factor.

Convert marine and aviation speed in knots to mph and km/h using the exact 1,852-metre nautical mile, distinct from the statute mile.

Convert energy between joules, kilocalories (food Calories), and BTU using their exact defining constants — and see why a small calorie is not a food Calorie.

Convert a US tablespoon (14.79 mL) against the UK and Australian metric tablespoons (15 mL and 20 mL) before scaling a recipe across regions.

Convert Fahrenheit to Celsius with the exact linear formula, then see why gas mark uses a separate rounded lookup table instead of that formula.

Convert an event’s local start time to a shared UTC instant before subtracting, so a countdown across time zones lands on the right hour.

Apply the exact Gregorian rule — divisible by 4, except centuries, except centuries divisible by 400 — instead of the incomplete “divisible by 4” shortcut.

Apply Zeller’s congruence — a fixed arithmetic formula, not a lookup table — to find the weekday of any Gregorian date by hand.

Find an ISO 8601 week number from the rule that week 1 contains the year’s first Thursday, and see why some years run to week 53.

Convert a letter grade to grade points on the common unweighted 4.0 scale, and see why a plus/minus scale is a separate school-specific choice.

Calculate a percentile rank as the share of a comparison group scoring at or below a given raw score, and see why it depends entirely on that group.

Estimate how much a tyre pressure reading rises with temperature using the roughly 1 psi per 10°F rule, and check it against the ideal gas law.

Divide a trip’s total fuel and toll cost evenly among passengers and distribute the leftover cents so the shares add back to the exact total.

Replace granulated sugar with honey using the 3/4-cup ratio, then adjust liquid, baking soda, and oven temperature for honey’s moisture and acidity.

Use a minutes-per-pound range to plan a roast’s oven time, then confirm the USDA’s minimum safe internal temperature with a food thermometer rather than the clock.

Convert a target pounds-of-nitrogen-per-1,000-square-feet application rate into an actual bag weight, using the fertilizer’s printed nitrogen percentage.