In this example, increasing monthly retainer fee from 3,000 to 3,300 changes the result from 33.3333 hours per month to 37.3333 hours per month. All other inputs stay the same.
Try this with your own numbersFormula and worked example
Available hours = (retainer × (1 − margin) − other delivery costs) / hourly cost
| Input | Example |
|---|---|
| Monthly retainer fee | 3,000 |
| Non-labour monthly delivery cost | 300 |
| Target contribution margin | 40 % |
| Loaded delivery hourly cost | 45 |
| Calculated result | 33.3333 hours per month |
Choose reliable inputs
Plan materials, travel and crew time before committing to a price. For this electrical service calculation, use a completed job sheet, supplier quotes and a crew schedule and a consistent definition of one service visit.
- Components: include the portion belonging to this service visit or reporting period, and record where the value came from.
- Diagnostic time: include the portion belonging to this service visit or reporting period, and record where the value came from.
- Callout overhead: include the portion belonging to this service visit or reporting period, and record where the value came from.
Use the same currency for every money input. Do not mix weekly costs with monthly revenue, or count the same expense in both an itemised cost and allocated overhead.
How many hours can a retainer support?
A negative answer means other costs already exceed the chosen cost envelope. Fixed overhead is not included unless entered.
What if this input changes?
If monthly retainer fee rises by 10%, this example changes by 4 hours per month. This shows the effect of one change; it does not predict what will happen.
Open the calculator, save your baseline, then change the input supported by the least reliable estimate in your records. Compare both scenarios before committing to a price or capacity promise.