Cost Performance Index Calculator
A project can report steady progress and still be quietly burning its budget. The Cost Performance Index, or CPI, exposes that gap in one ratio: it compares the budgeted value of the work you have actually finished with what that work really cost.
Enter your earned value and your actual cost below. The calculator returns your CPI to two decimal places, what each dollar spent is earning back, and a plain verdict on whether you are over, on, or under budget.
This is an earned value metric, not a unit price. It says nothing on its own about schedule, scope, or quality, so read it alongside your Schedule Performance Index and your delivery checks.
Quick Answer
Key takeaways
- CPI is earned value divided by actual cost. It is a ratio, not a dollar amount and not a cost per unit.
- Below 1.00 means over budget, exactly 1.00 means on budget, above 1.00 means under budget for the work completed.
- Get EV from progress, not from spending: EV equals percent complete multiplied by the total budget (Budget at Completion).
- CPI feeds the standard forecast EAC equals BAC divided by CPI, which is why a small CPI slip moves the final bill a lot.
- Quick Answer
- How to use the cost performance index calculator
- Where earned value comes from
- The CPI formula and how to read the result
- Worked example: a website build running over budget
- CPI is not SPI, and neither one is the whole story
- What competing calculators miss
- Frequently Asked Questions
- Add This Calculator to Your Website
- References
How to use the cost performance index calculator
Enter earned value first, then actual cost for the same work and the same reporting date, and press Calculate CPI.
The date match matters more than the arithmetic. EV must describe the work completed up to today, and AC must be everything spent to complete that same work up to today. If you include next month invoices in AC but not their work in EV, the CPI will look worse than reality. If you count work as earned before it has passed its acceptance checks, the CPI will look better than reality. Project teams usually pull AC from timesheets, supplier invoices, and expense records, and pull EV from the work breakdown structure, where each task carries a budgeted cost and a percent complete. Teams that allocate shared costs across projects often sanity check that AC build-up against an activity based costing calculator so overhead is not dumped into one project by mistake.
Where earned value comes from
Earned value is the budgeted cost of work performed, often shortened to BCWP in older earned value texts.
The working formula is EV equals percent complete multiplied by Budget at Completion (BAC), the total approved budget for the project or work package. A work package budgeted at $120,000 that is genuinely 40% complete has earned $48,000 of value, whether you have spent $30,000 or $60,000 getting there. That independence is the whole point: EV is anchored to the plan, AC is anchored to the bank account, and CPI compares the two anchors. According to PMI guidance on earned value forecasting, these cumulative EV and AC figures are also the inputs behind the standard completion forecasts used in formal project reviews.
The CPI formula and how to read the result
CPI equals EV divided by AC, reported here to two decimal places, exactly as the Project Management Institute defines the index in its earned value practice.
Read the result as value per dollar. A CPI of 0.80 means each dollar spent bought 80 cents of planned value. A CPI of 1.25 means each dollar bought $1.25 of planned value. The companion figure, cost variance, states the same story in dollars instead of a ratio: CV equals EV minus AC, so the worked example below has a cost variance of negative $12,000. Ratios travel well between projects of different sizes, which is why executives track CPI, while project accountants often track the dollar variance alongside it using a cash flow variance calculator to separate timing effects from real overruns.
| CPI result | Reading | Typical response |
|---|---|---|
| Below 0.85 | Serious overrun | Investigate root cause now, re-forecast the final cost, and agree corrective action with the sponsor. |
| 0.85 to 0.99 | Over budget | Find the work packages driving the slip and tighten cost control before it compounds. |
| 1.00 | On budget | Hold course and keep measuring each period. One reading is a snapshot, not a trend. |
| 1.01 to 1.15 | Under budget | Verify the progress claims are real, then protect whatever is working. |
| Above 1.15 | Well under budget | Check for padded estimates, skipped scope, or costs booked to the wrong project before celebrating. |
Those bands are working conventions used by many project offices, not thresholds set in the PMBOK Guide. The guide fixes the formula and the over, on, and under reading at 1.00; how much deviation triggers action is a governance choice for your organisation.
Worked example: a website build running over budget
The calculator is pre-filled with this case, and every figure below is what it returns or follows directly from its result.
A website build has a total budget (BAC) of $120,000. At the mid-project review the team has completed work worth 40% of that budget, but the invoices and timesheets tell a heavier story.
| Step | Calculation | Result |
|---|---|---|
| Earned Value | 40% x $120,000 | $48,000 |
| CPI | $48,000 / $60,000 | 0.80 |
| Cost Variance | $48,000 – $60,000 | -$12,000 |
| Forecast final cost (EAC) | $120,000 / 0.80 | $150,000 |
That forecast is the number sponsors actually argue about. If cost efficiency stays at 0.80 for the remaining work, the project needs about $150,000 to finish, which is $30,000 over its approved budget. Surfacing that gap at the halfway point, while scope, staffing, or sequencing can still change, is exactly the job CPI exists to do. Teams weighing whether to crash the schedule to recover time should also price that decision, since faster work usually costs more, and a cost of delay calculator helps compare the delay cost with the recovery cost.
CPI is not SPI, and neither one is the whole story
CPI measures cost efficiency only. Its schedule sibling, the Schedule Performance Index, is SPI equals EV divided by Planned Value (PV), where planned value is the budgeted cost of the work that was scheduled to be done by now.
A project can post a healthy CPI of 1.10 while running late, if the small amount of work it did manage to finish came in cheap. It can also post a CPI of 0.80 while running ahead of schedule, if overtime and expediting bought that speed. That is why status reports pair the two indices, and why a combined forecast sometimes divides BAC by CPI multiplied by SPI when both cost and schedule pressure are expected to shape the remaining work. CPI also says nothing about whether the finished work is any good. Accepted, inspected work should count toward EV; work that will need rework should not, or the index will flatter the project until the rework bill arrives. When a CPI slip forces a wider rethink of scope, timing, and funding, a structured budget impact calculator is a useful next step for sizing the options.
What competing calculators miss
Most CPI tools reviewed for this page fall into two camps, and each camp leaves a real gap.
Miss 1: the mega-suite that buries the simple question. Full earned value suites ask for BCWS, BCWP, and ACWP under their older names, then return a wall of indices at once. That is powerful for trained EVM analysts, but a project lead who just needs CPI from EV and AC has to translate terminology before dividing two numbers. This page keeps the two modern inputs, EV and AC, and labels them in plain language with examples.
Miss 2: the bare ratio with no forecast. Several single-purpose tools return 0.80 and stop. The decision that follows from 0.80 is the forecast: at that efficiency the final cost is BAC divided by CPI. This article works that forecast in full, with the $120,000 to $150,000 example, and states the assumption behind it, which is that current cost efficiency continues. If the overrun came from a one-off cause that will not repeat, the honest forecast is different, and the FAQ below covers that case.
Miss 3: a wrong formula variant still circulating. At least one tutorial presents CPI as (EV / AC) x 100 and reports results like 83.33. That is a percentage mislabelled as an index. The standard index is a plain ratio, so 0.83, not 83.33, and every interpretation band, forecast formula, and PMP exam answer depends on keeping it that way.
Frequently Asked Questions
A CPI of 1.00 or above is good: the project is on or under budget for the work completed. Below 1.00 means over budget, and many project offices investigate in earnest once CPI falls under about 0.85, though that trigger is an organisational choice, not a standard threshold.
Multiply percent complete by the total budget: EV = percent complete x Budget at Completion (BAC). A project that is 40% complete on a $120,000 budget has an earned value of $48,000, regardless of what it has spent so far.
CPI = EV / AC measures cost efficiency: value earned against money spent. SPI = EV / PV measures schedule efficiency: value earned against value planned by now. A project can be under budget (CPI above 1) and behind schedule (SPI below 1) at the same time.
CPI is undefined, because dividing by zero has no answer. In practice an actual cost of zero at a review point almost always means costs have not been booked yet, so record the invoices and timesheets first and then calculate.
The standard forecast is Estimate at Completion: EAC = BAC / CPI, assuming current cost efficiency continues. At a CPI of 0.80, a $120,000 budget forecasts to $150,000. If the overrun was a one-off event, use EAC = AC + (BAC – EV) instead, which assumes the remaining work runs at planned efficiency.
They report the same comparison in different units. Cost variance is a dollar amount, CV = EV – AC, so $48,000 minus $60,000 is -$12,000. CPI is the ratio, EV / AC, so 0.80. The ratio compares projects of different sizes fairly; the dollar figure shows the size of the hole.
At every regular reporting period, typically weekly or monthly, and at each major milestone. A single CPI is a snapshot; the trend across periods is what shows whether corrective action is working or the overrun is compounding.
No. A high CPI can also mean estimates were padded, scope was quietly dropped, quality checks were skipped, or costs were booked to another project. Verify that the work counted as earned is genuinely complete and accepted before treating a high CPI as good news.
Add This Calculator to Your Website
References
- Project Management Institute, “Applications: Extensions: Earned Value Analysis Forecast Outcomes” – the EAC = BAC / CPI forecast and variance at completion formulas used in this article. PMI: Earned Value Analysis Forecast Outcomes. Accessed October 2026.
- Atlassian, “How to Calculate Cost Performance Index” – CPI definition, the over, on, and under budget readings at 1.0, and data sources for EV and AC. Atlassian: Cost Performance Index. Accessed October 2026.
- PM Study Circle, “Cost Performance Index (CPI) in Project Management” – worked CPI example building EV from percent complete and BAC. PM Study Circle: Cost Performance Index. Accessed October 2026.
- Plaky, “Cost Performance Index (CPI): What It Is and How to Calculate It” – CPI formula, EV as percent complete multiplied by total budget, and the CPI to EAC relationship. Plaky: Cost Performance Index. Accessed October 2026.
- Nulab, “The Project Manager Guide to Cost Variance” – cost variance and schedule variance formulas and their pairing with CPI and SPI. Nulab: Guide to Cost Variance. Accessed October 2026.
- Project Management Academy, “The EAC Formula: PMP Questions and Insights into Project Budgets” – the four EAC formula variants and when each assumption applies. Project Management Academy: The EAC Formula. Accessed October 2026.