Schedule Forecast Calculator
Calculate Schedule Forecast step by step: Forecast duration = planned duration ÷ SPI.
In short
Formula: Forecast duration = planned duration ÷ SPI.
What this calculator does
Calculate Schedule Forecast step by step: Forecast duration = planned duration ÷ SPI. Worked example, questions and limitations included.
Use it to turn Planned duration, Time elapsed, Schedule performance index (SPI) into a checked result you can compare, copy, or rerun with different assumptions.
The page shows the formula, a numeric worked example, and the assumptions that affect this project estimation calculation.
Inputs and what they mean
- Planned duration
- — number value.
- Time elapsed
- — number value.
- Schedule performance index (SPI)
- — number value.
How to use it
- Enter each value in the unit shown next to the box (use the unit converter first if your numbers are in other units).
- Check the breakdown to see every intermediate step.
- Read the limitations before relying on the result.
Formula
A common EVM schedule estimate (independent estimate of duration at completion, IEAC(t)).
Assumes the current schedule efficiency continues for the rest of the work.
Earned schedule (Lipke, 2003) gives better late-project forecasts; this tool uses the simpler SPI method.
Forecast duration = planned duration ÷ SPI.
Inputs used: Planned duration, Time elapsed, Schedule performance index (SPI).
Worked example
40-week plan, SPI 0.875
- 40 ÷ 0.875 = 45.7 weeks.
- Slip = 5.7 weeks; remaining = 33.7 weeks.
Reading the result
The headline figure is the main answer. Any breakdown underneath shows the parts that make it up, so you can check the working and see what changes when you adjust an input.
Limitations and assumptions
- Ignores the critical path.
- Degrades in the final third of the project.
- Assumes constant efficiency.
- Results are planning estimates built only from the figures you enter. They are not a guarantee of cost, schedule or delivery, and they do not replace your organisation's own project controls.
- The result depends on the values you enter for this schedule forecast calculator; it does not supply missing rates, rules, prices, dates, or assumptions for you.
Common questions
Why is this only a rough guide?
SPI measures money-weighted progress, not the critical path. A late critical task can delay the finish more than SPI suggests.
What is earned schedule?
A method that converts EV into time, fixing SPI's drift toward 1 at the end. It needs the full time-phased baseline.
Where does SPI come from?
SPI = EV ÷ PV; see the SPI calculator. Check the formula, example, and limitations on this page before using the result for a real project estimation decision.
How do I use the Schedule Forecast Calculator?
Enter the required values for Planned duration, Time elapsed, Schedule performance index (SPI). The calculator applies the formula on this page and shows the main result with any supporting breakdown so you can check the arithmetic.
What formula does the Schedule Forecast Calculator use?
Forecast duration = planned duration ÷ SPI. The visible formula section above lists the calculation path and the edge cases the page handles, so the result can be checked without relying on the form alone.
Can the Schedule Forecast Calculator be used for exact decisions?
Use it as a calculation aid, not as a substitute for checking the underlying rule, contract, policy, or professional advice that applies to your situation. When a result depends on local rules, personal details, prices, or dates, enter those values yourself and confirm them before acting.
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