Everyday calculation guides — page 3 of 4

Read a pressure label by retaining its unit, pressure reference, and practical rounding before converting.

Separate a change measured in percentage points from a relative percentage increase.

Calculate a fraction of an amount and express the same share as a percentage without mixing the two forms.

Choose a ratio, rate, or proportion by checking whether the compared quantities have units.

Use a weighted average when observations represent unequal credits, quantities, or group sizes.

Select population or sample standard deviation from what the observed values represent, not from which number looks better.

Use the issuing school’s conversion and credit rules before translating a percentage into GPA.

Write a number as a coefficient times a power of ten and move the decimal in the direction the exponent states.

Use BMI as an adult screening measure of weight relative to height, not as a diagnosis or direct body-fat measurement.

Distinguish resting-energy estimate, total daily expenditure estimate, and an intake-versus-expenditure gap.

Derive a fixed car loan payment from principal, interest rate, and term with the standard amortization formula.

Follow how a fixed car loan payment splits between interest and principal as the balance falls.

Solve the amortization formula backward from a monthly budget to the loan amount it supports.

Turn two real calendar dates into a day count by converting each to a day-of-year number first.

Add a day count across month and year boundaries and see why it rarely matches adding whole months.

Find a precise age by checking whether the birthday has occurred this year before subtracting.

Combine a day count with a clock-time difference to find total elapsed hours across multiple days.

Check why BMI uses one formula for adult men and women and where that shared formula runs into limits.

See how NICE recommends lower BMI thresholds for some ethnic groups than the general UK/WHO brackets.

Understand why the same raw BMI number is classified by age- and sex-specific percentile for ages 2–19.