Money + Math

Money Math Center

Learn the math behind everyday money decisions. Work with budgets, percentages, interest, loans, savings, prices, wages, profit, markup, margin, taxes, tips, inflation and more—with formulas and plain-language explanations.

Financial Calculators →Calculators With Steps →Time Math →Learning Paths →

Budget & Cash Flow Math

See where money goes and turn totals into useful percentages.

Monthly Budget

Monthly income

Monthly expenses

$900 left · 22.5% savings rate

Leftover = income − expenses. Savings rate = leftover ÷ income × 100.

50/30/20 Guide

Take-home monthly income

Needs $2,000 · Wants $1,200 · Saving/debt $800

A planning rule, not a requirement: 50% needs, 30% wants, 20% saving/debt goals.

Percentages in Money

Percent math appears in sales, taxes, tips, raises, interest, investment returns and price changes.

Discount + Sales Tax

Original price

Discount %

Sales tax %

Discount $20 · Tax $5.60 · Final $85.60

Discount first, then tax the discounted price.

Tip + Split Bill

Bill

Tip %

People

Tip $16 · Total $96 · $48 each

Savings & Interest Math

Simple Interest

Principal

Annual rate %

Years

Interest $150 · Total $1,150

I = P × r × t.

Compound Growth

Starting amount

Annual rate %

Years

Compounds/year

Future value $1,647.01

A = P(1 + r/n)^(nt).

Savings Goal

Goal

Already saved

Months remaining

Save about $416.67 per month

Rule of 72 Estimate

Annual return %

About 9 years to double

Approximation: 72 ÷ annual percentage rate.

Loans & Debt Math

Loan Payment

Loan amount

APR %

Term months

Monthly payment about $396.02

Uses the standard amortizing loan payment formula.

Debt Payoff — No New Interest

Balance

Monthly payment

About 17 months

Simple learning model: balance ÷ payment. Real interest-bearing debt takes longer.

Business Money Math

Markup & Margin

Cost

Selling price

Profit $40 · Markup 66.67% · Margin 40%

Markup uses cost as the base. Margin uses selling price as the base.

Break-Even Units

Fixed costs

Price/unit

Variable cost/unit

250 units

Fixed costs ÷ (price − variable cost).

Pay, Wages & Time Math

Hourly ↔ Annual Pay

Hourly rate

Hours/week

Paid weeks/year

Weekly $1,000 · Annual $52,000

Raise Calculator

Current pay

Raise %

Increase $2,000 · New pay $52,000

Shopping & Comparison Math

Unit Price

Total price

Quantity

$0.75 per unit

Percent Change

Old value

New value

25% increase

Inflation & Purchasing Power

Future Cost Estimate

Cost today

Inflation %/year

Years

About $134.39 after 10 years

Future cost = present cost × (1 + inflation rate)^years.

Purchasing Power

Money today

Inflation %/year

Years

Equivalent buying power about $744.09

Money Math Lessons

Core ideas that connect practical money decisions to arithmetic, percentages, algebra and exponential growth.

Percent means “per hundred”7% tax means $7 for every $100 of taxable price.

APR is an annual rateLoans often convert annual percentage rates to monthly rates for payment calculations.

Compound growth is exponentialGrowth applies to the previous balance, not only the original amount.

Markup ≠ marginMarkup compares profit to cost; margin compares profit to selling price.

Unit price enables fair comparisonsDivide price by quantity to compare different package sizes.

Time changes money mathInterest, inflation, wages and savings all depend on how long the money is working.

Quick Money Math Formula Reference

Percent amountAmount × percent ÷ 100

Simple interestI = P × r × t

Compound growthA = P(1 + r/n)^(nt)

ProfitSelling price − cost

Margin %Profit ÷ selling price × 100

Markup %Profit ÷ cost × 100

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