The maths, explained
Extra work should be priced from a clear scope before it becomes a dispute. Add the extra labour, materials, subcontractors and allocated overhead, then apply your target margin to the selling price. This calculator provides a cost-based scenario only. The signed agreement may require a different markup, supporting records, notice period or written approval before work starts.
Change order price = (labor + materials + subcontractors + overhead) ÷ (1 − target margin)
A worked example
The worked example below uses the calculator’s initial inputs and the same calculation function as the live interface. Replace example amounts with your own records; changing currency does not convert the numbers. Physical reference dimensions are stored in metric units, and the interface can display their imperial equivalents. Rounded display values may differ slightly from the full precision retained in the calculation.
| Your inputs | Editable example — not a market quote |
|---|---|
| Additional labor cost (CAD) | 1,800 |
| Additional materials (CAD) | 1,200 |
| Additional subcontractors (CAD) | 500 |
| Change-order overhead (CAD) | 350 |
| Target profit margin (%) | 20 |
| Suggested change order price | $4,812.50 |
| Additional direct and overhead costs | $3,850.00 |
| Expected gross profit | $962.50 |
Assumptions used
All cost inputs must cover the same period and use one currency. Employer charges and overhead remain business-specific inputs. Defaults are educational scenarios, not market averages. This model does not determine tax obligations or contractual rights. Ratios with a zero denominator are marked as undefined rather than presented as a misleading zero percent.
On your side of the Atlantic
Canadian projects often mix metric drawings with imperial products. Confirm the supplier’s units and the applicable GST, HST or PST treatment for your province.
Common mistakes
Include only additional costs rather than the original contract again. Do not use a margin formula when the contract specifies a markup. Document approval and any programme effects separately from this price estimate.
Before ordering or tendering, compare at least two scenarios: your base takeoff and a realistic alternative. Keep assumptions with the result so differences are explainable. Save an estimate only on a suitable device. A shared link contains the entered values, so avoid including confidential information. An exported estimate is a snapshot and does not update when a supplier changes a price.
Questions from the jobsite
What does this calculator measure?
What is the first mistake to avoid?
Are the example prices supplier quotes?
Which tax assumptions apply in Canada?
How should I handle waste and contingency?
Can I use the result in a tender or estimate?
Planning estimate only. Not a substitute for a professional quote or local building-code compliance. Verify quantities, tax treatment and contract requirements.
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