UK KS3 Maths · Year 8 · Financial Mathematics

Interest Calculator

Explore simple interest, compound interest, depreciation and reverse calculations — with full step-by-step working.

Simple Interest

A = P × (1 + r/100 × t)

Compound Interest

A = P × (1 + r/100)ᵗ

Principal (£)

Interest Rate (%)

Years

yrs

Simple — after 10 yrs

—

Compound — after 10 yrs

—

Extra from compounding

—

Year 1Year 10

📖 Step-by-step working

Find how much interest accumulates between two specific years — useful for comparing saving milestones.

Principal (£)

Rate (%)

From year

To year

Value at year 5

—

Value at year 10

—

Interest (yrs 5–10)

—

📖 Step-by-step working

Given a starting amount, a final amount, and the number of years, work out what interest rate was applied.

Principal (£)

Final Amount (£)

Years

yrs

Interest rate found

—

Total interest earned

—

📖 Step-by-step working

How long does it take to reach a target amount? Work backwards to find the number of years needed.

Principal (£)

Target Amount (£)

Rate (%)

Time needed (exact)

—

Rounded up to whole years

—

📖 Step-by-step working

Depreciation means a value decreases each year — the same maths as compound interest but going downward. Common for cars, technology, and equipment.

Starting Value (£)

Depreciation Rate (%)

Years

yrs

Value after 5 years

—

Total value lost

—

% of original remaining

—

Year	Value (£)	Lost so far (£)

📖 Step-by-step working

📐 Formula reference sheet

Simple amount: A = P(1 + rt/100)

Simple interest: I = Prt/100

Compound amount: A = P(1 + r/100)ᵗ

Find rate (comp): r = ((A/P)^(1/t) − 1) × 100

Find years (comp):t = log(A/P) ÷ log(1+r/100)

Depreciation: V = P(1 − r/100)ᵗ

P = principal · r = rate (%) · t = years · A/V = final amount/value