How to build a simple monthly sales forecast

Choose the driver that matches the business

A service business can forecast billable hours times realized hourly rate. A product business can use units times net selling price. A subscription business needs starting subscribers, additions, churn and revenue per subscriber. Avoid treating orders, customer counts and billable hours as interchangeable units.

Build a three-month example

Start a product forecast at 100 units in month one and assume 10% monthly volume growth, with price fixed at 50. The model gives 100, 110 and 121 units, producing revenue of 5,000, 5,500 and 6,050. Total revenue is 16,550. This is a compounding assumption: adding ten units each month would instead give 100, 110 and 120.

Connect the forecast to capacity

If capacity is 115 units in month three, the unconstrained forecast of 121 cannot be delivered without a change. At 115 units and the same price, revenue is 5,750. Record whether you will cap sales, outsource delivery, add capacity or revise timing. Any extra costs belong in the profit forecast.

Compare a baseline with a downside

A flat-volume case at 100 units each month produces 15,000 over the same three months. It is 1,550 below the growth case. Label both as scenarios until there is evidence supporting one. Seasonality, pipeline conversion and repeat purchasing may provide better drivers than a convenient constant growth rate.

Replace assumptions with actual results

At month end, keep the original plan for comparison and record the observed volume and price. Explain the difference before changing future months. This avoids rewriting history to make performance look accurate. Forecast revenue is not a profit forecast or a cash collection forecast; model costs and payment timing separately.

Choose the driver that fits the business

A product business might use units sold × realized price. A service business might use available delivery hours × billable utilization × hourly rate. A project business might schedule signed work by delivery month. A subscription business needs opening customers, churn and new additions rather than a simple repeated sales growth rate.

Choose a driver you can observe and influence. An annual revenue target divided by twelve is a useful reference line, but it does not explain seasonal demand, hiring delays or the pipeline needed to reach it.

Separate booked work from assumptions

Label the part of the forecast supported by signed orders or contracts, then show the additional assumed sales. A pipeline opportunity is not equivalent to a signed contract. Even signed work can have cancellation or delivery risk, so document the basis rather than presenting every future month as certain.

For a service example, 160 available hours at 60% billable utilization and a rate of 75 produce 7,200 revenue. At 50% utilization, revenue falls to 6,000. Availability and utilization are distinct assumptions: adding a person does not guarantee enough billable work.

Update the forecast without rewriting the original plan

Keep the original plan as a comparison point. After a month closes, record actual sales and costs, explain the variance and update only the forward-looking assumptions in a separate forecast. Otherwise, replacing every missed target with the actual result erases the evidence you need to learn.

Review the biggest driver first. Was volume below plan, did realized price change, or was delivery more expensive? Then check whether the explanation is temporary or likely to continue. A revised forecast is a new estimate with a date and reasoning, not a promise to recover every previous shortfall.

Try the numbers yourself

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