Project Management guide · Cost and resources
Cost Estimation in Project Management: Methods and Examples
Cost estimation is the process of predicting how much money a project's work will need. A good estimate states not just a number but its range of accuracy, the assumptions behind it and how it was produced.
Key takeaways
- Choose the estimating method to match how much you know about the work.
- Always quote a range and document the basis of estimate.
- Three-point and PERT estimates make uncertainty visible.
- Re-estimate as the project progresses and knowledge improves.
Main estimating methods
| Method | How it works | Best used when |
|---|---|---|
| Analogous (top-down) | Uses the actual cost of a similar past project, adjusted for differences | Early on, with little detail |
| Parametric | Multiplies a unit rate by a quantity, such as cost per square metre | Work is repeatable and good historical rates exist |
| Bottom-up | Estimates each work package and adds them up | The WBS is well defined |
| Three-point | Uses optimistic, most likely and pessimistic values | Uncertainty is significant |
| Vendor bid analysis | Compares supplier quotes | Work will be bought in |
| Expert judgment | Draws on people who have done similar work | Alongside any other method |
Estimate accuracy ranges
Estimates become more accurate as the project is better understood. Commonly quoted ranges are:
| Estimate class | Typical range | When |
|---|---|---|
| Rough order of magnitude (ROM) | -25% to +75% | Initiation, ideas and feasibility |
| Budget estimate | Around -10% to +25% | Planning, once scope is outlined |
| Definitive estimate | Around -5% to +10% | Detailed planning, once the WBS is complete |
Always quote the range, not just a single figure, so decision makers understand the uncertainty.
How to estimate project costs
- Confirm the scope and break it down into work packages.
- Identify the resources each package needs: people, equipment, materials, services.
- Choose a method for each package and apply current rates.
- Add the cost of risk responses and a contingency reserve based on the risk register.
- Include indirect costs, licences, travel, training and inflation for long projects.
- Document the basis of estimate: assumptions, sources, exclusions and the accuracy range.
- Review with the people who will do the work and with finance.
Worked examples
Parametric: an office fit-out costs about $1,200 per square metre. For 250 square metres: 1,200 x 250 = $300,000.
Analogous: a previous 40-screen app cost $80,000, or $2,000 per screen. A new 50-screen app of similar complexity: 50 x 2,000 = $100,000, before adjusting for differences.
Three-point (PERT): optimistic $8,000, most likely $12,000, pessimistic $22,000. PERT estimate = (8,000 + 4 x 12,000 + 22,000) / 6 = 78,000 / 6 = $13,000. The triangular average would be (8,000 + 12,000 + 22,000) / 3 = $14,000.
Bottom-up: design $18,000 + build $54,000 + testing $16,000 + training $7,000 = $95,000, plus a contingency reserve of $9,500 (10%) = $104,500.
Tips for better estimates
- Estimate effort first, then convert to cost; see effort estimation.
- Have the people who will do the work estimate it.
- Keep a library of actual costs from past projects.
- Separate contingency from the base estimate so it can be managed openly.
- Re-estimate at each phase as knowledge improves (progressive elaboration).
- Watch for optimism bias and pressure to hit a target number.
Free templates for this topic
Frequently asked questions
What is cost estimation in project management?
It is predicting the money needed to complete the project's work, stated with a range of accuracy and the assumptions behind it.
What are the four main cost estimating techniques?
Analogous, parametric, bottom-up and three-point estimating are the most widely used.
What is the most accurate cost estimating method?
Bottom-up estimating is usually the most accurate because it is based on detailed work packages, but it takes the most time.
What is a rough order of magnitude cost estimate?
A very early estimate, commonly quoted as -25% to +75% accurate, used to decide whether an idea is worth pursuing.
What is the PERT formula for cost estimating?
PERT estimate = (optimistic + 4 x most likely + pessimistic) / 6. It weights the most likely value most heavily.
What is the difference between analogous and parametric cost estimating?
Analogous estimating scales the total from a similar past project. Parametric estimating multiplies a unit rate by a quantity, such as cost per metre.
What is a basis of estimate in cost estimation?
Supporting documentation explaining how an estimate was produced: assumptions, sources, rates, exclusions, risks and the accuracy range.
Should contingency be included in the cost estimate?
Contingency for identified risks is added to the estimate to form the cost baseline, but it should be shown separately rather than hidden in task estimates.
Who should prepare cost estimates?
The people doing the work, supported by the project manager, cost specialists and finance, ideally reviewed against historical data.
How does cost estimation work for Agile projects?
Agile teams often estimate cost per sprint (team cost x number of sprints) and combine it with relative estimates of scope; see story points.
Why is cost estimation on projects often wrong?
Common reasons are unclear scope, optimism, missing items such as indirect costs, unmanaged risks and pressure to quote a low number.