Finance⏱ 5 min read
How to Calculate Payback Period for Any Investment
The payback period tells you how long it takes to recoup an investment. Here is the formula for both equal and unequal cash flows, its limitations, and when to use it alongside NPV.
The payback period is one of the simplest capital budgeting tools: how many years before you get your money back? Its simplicity is both its strength and its main weakness.
Simple Payback Period (Equal Annual Cash Flows)
Payback Period = Initial Investment / Annual Cash Flow
Example: Solar panels
Cost: £8,000 installed
Annual energy saving: £900/year
Payback = £8,000 / £900 = 8.9 years
This is valid only when cash flows are roughly equal each year.
Solar panels degrade slightly each year — see unequal method below.
Cumulative Payback (Unequal Cash Flows)
Track cumulative cash flows until the balance goes positive.
Year | Annual CF | Cumulative CF
0 | -£8,000 | -£8,000
1 | £900 | -£7,100
2 | £920 | -£6,180
3 | £940 | -£5,240
4 | £950 | -£4,290
5 | £960 | -£3,330
6 | £970 | -£2,360
7 | £980 | -£1,380
8 | £990 | -£390
9 | £1,000 | +£610
Payback between years 8 and 9.
Precise: 8 + (390/1000) = 8.39 years
Discounted Payback Period
Adjusts each year's cash flow for the time value of money.
More accurate but more complex.
Discount rate: 5% (opportunity cost of capital)
Year | Annual CF | Discount Factor | PV of CF | Cumulative PV
0 | -£8,000 | 1.000 | -£8,000 | -£8,000
1 | £900 | 0.952 | £857 | -£7,143
2 | £920 | 0.907 | £834 | -£6,309
3 | £940 | 0.864 | £812 | -£5,497
4 | £950 | 0.823 | £782 | -£4,715
5 | £960 | 0.784 | £753 | -£3,962
6 | £970 | 0.746 | £724 | -£3,238
7 | £980 | 0.711 | £697 | -£2,541
8 | £990 | 0.677 | £670 | -£1,871
9 | £1,000 | 0.645 | £645 | -£1,226
10 | £1,000 | 0.614 | £614 | -£612
11 | £1,000 | 0.585 | £585 | -£27
12 | £1,000 | 0.557 | £557 | +£530
Discounted payback: ~11.05 years (vs 8.39 simple payback)
The gap shows what time value of money costs you.
Payback Period vs NPV
Payback ignores cash flows AFTER the payback period.
Two projects, £10,000 investment each:
Project A: £5,000/year for 2 years, then nothing
Project B: £2,500/year for 10 years
Payback period: A = 2 years, B = 4 years → A looks better
NPV at 10%: A = -£327 (destroys value!), B = +£5,361
The project with the faster payback has negative NPV.
Always use NPV for economic decision-making.
Use payback only as a liquidity/risk screen, not a value measure.