The payback period is the number of years it takes for a project’s cumulative net benefits to recover its first cost. A project that costs $100,000 upfront and saves $25,000/year pays back in 4 years.
For constant annual benefits:
For variable benefits, cumulate them year by year and find the first year where the cumulative total exceeds the first cost. (Fractional years are sometimes interpolated within the final partial year.)
Decision rule. Set a hurdle payback period, commonly 2 to 4 years for industrial capital projects, longer for infrastructure. Accept projects with payback at or under the hurdle; reject longer ones.
Payback is the simplest project-evaluation method, which is both its appeal and its main weakness.
Strengths. Easy to calculate, easy to communicate, works without estimating long-horizon cash flows or picking a MARR. Captures liquidity concerns: a short payback returns cash to the business quickly. Useful as a secondary screen alongside PW/IRR.
Weaknesses.
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Ignores the Time value of money. A dollar received in year 4 is treated the same as a dollar received in year 1. This is mathematically wrong and gives the wrong ranking when time-value is material.
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Ignores cash flows beyond payback. A project that pays back in 3 years and then generates nothing is treated identically to one that pays back in 3 years and then generates $20,000/year for the next 10. The second is obviously far better.
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No risk-adjustment. All cash flows are treated as certain.
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Encourages short-termism. Strict payback hurdles bias the firm toward small quick-payback projects and against large strategic investments (R&D, new facilities) with long ramps.
The discounted payback period partly addresses the time-value issue: it uses discounted cash flows (the same ones that go into PW) instead of nominal flows. Discounted payback is always longer than simple payback, by the time-value gap.
Payback is usually a complementary metric, not a primary decision tool: “yes, this is the PW-best project, but it pays back in 8 years, do we have the patience?” It’s most defensible when liquidity is the binding constraint, or when forecasts beyond a short horizon are too uncertain to trust. For the rigorous alternatives see Present worth method and Internal rate of return.