Every small business runs on the same uncomfortable question: "How much revenue are we actually going to bring in over the next quarter?" A sales forecast turns that guess into a number you can plan payroll, inventory, and ad spend around. This calculator projects your next 3 to 24 months of sales by fitting a straight trend line through your last three completed monthly totals and extending it forward, with an optional seasonality adjustment for peaks and slowdowns.
The model is deliberately simple — ordinary least-squares linear regression on three points, then a flat seasonality multiplier. That is exactly the method recommended by the Australian Government Business portal and used as the baseline inside more sophisticated tools like Meta's open-source Prophet library. It will not predict a product launch or a viral TikTok moment, but it will give you a defensible baseline number in seconds.
How to Use
- Enter your Sales — Month -2 figure. This is the earliest of your three reference months. Use gross sales for the month, not profit.
- Enter Sales — Month -1, the middle reference month.
- Enter Sales — Most Recent Month, the latest fully closed month. Do not use a partial or in-progress month — it will pull the trend line off.
- Set the Forecast Horizon to how many months ahead you want to plan. Six months is the default and the most common planning window; twelve months works for annual budgets.
- Add a Seasonality Adjustment if the forecast window falls in a known peak or trough. For example, a retail store in November might use +20% for the holiday bump; a pool supply shop forecasting December might use -30%. Leave it at 0 if your business runs roughly flat all year.
- Read the result card: the trend tells you how many dollars per month your line is adding, the next-month and final-month forecasts bracket the range, and the total forecast is the sum you should plan against.
Because the engine fits a line through only three points, one anomalous month (a one-off B2B contract, a weather shutdown) will swing the result hard. If any of your three months is unusual, consider replacing it with a smoothed average before you project.
The Linear Trend Method
Given three monthly sales totals y1, y2, y3 for months 1, 2, 3, the ordinary least-squares slope on points (1, y1), (2, y2), (3, y3) collapses to a simple closed form:
slope = (y3 − y1) / 2
The mean of the three months is meanY = (y1 + y2 + y3) / 3, and the intercept is:
intercept = meanY − slope × 2
For any future month k (k = 4, 5, … up to 3 + N), the trend projection is:
trend(k) = intercept + slope × k
Then the seasonality adjustment multiplies the whole projection:
forecast(k) = trend(k) × (1 + seasonalityFactor / 100)
Worked example: months of $10,000, $12,000, and $14,000 give a slope of (14,000 − 10,000) / 2 = $2,000 per month. The mean is $12,000 and the intercept is $12,000 − $2,000 × 2 = $8,000. Month 4 lands at $8,000 + $2,000 × 4 = $16,000, month 5 at $18,000, and month 6 at $20,000. Over three months that is a $54,000 total, or $18,000 per month on average.
Why Three Months?
Three points is the minimum that defines a straight line while still letting the middle month exert some pull. With only two months the slope is just (y2 − y1), which is the same thing but with no check on whether the second month was an outlier. With six or twelve months you really want a weighted model that weights recent months more heavily and removes seasonality statistically — which is what Prophet and similar libraries do. For a quick baseline plan, three months of clean data plus a manual seasonality dial is the sweet spot between speed and defensibility.
Reading the Outputs
- Trend (Monthly Growth) — the slope, in dollars per month. A positive number means sales are growing; a negative number means they are shrinking.
- Next Month Forecast — the very next period on your calendar. This is the number to compare against next month's actuals to start tracking forecast accuracy.
- Last Projected Month — the far end of your horizon. Use it to see where the trend line lands if nothing changes.
- Average Monthly Forecast — total projected revenue divided by the number of months, useful for cash-flow planning.
- Total Forecast (Full Horizon) — the sum across all projected months. This is the revenue number to compare against your expenses and targets.
Examples
Example 1 — A steady e-commerce store. Three recent months came in at $10,000, $12,000, and $14,000. The owner sets a 6-month horizon with no seasonality. The trend adds $2,000 per month, so next month projects to $16,000 and month 6 to $26,000. The 6-month total is roughly $126,000, or $21,000 per month on average — a reasonable number to build an ad budget against.
Example 2 — Holiday peak. A gift shop has three baseline months of $20,000, $22,000, $24,000, and wants to plan November through January. It sets a 3-month horizon and a +25% seasonality adjustment for the holiday window. The raw trend would have given $26,000, $28,000, $30,000; after the 25% uplift the projected months become $32,500, $35,000, and $37,500, for a holiday quarter total of about $105,000. That is the number to pre-buy inventory against.
Example 3 — A slow summer. A construction subcontractor sees $90,000, $100,000, and $110,000 over the spring but knows July and August soften. It forecasts 3 months with a −15% seasonality factor. The unadjusted trend gives $120,000, $130,000, $140,000; adjusted, that becomes $102,000, $110,500, and $119,000, totaling roughly $331,500 — a more conservative cash-flow plan than the raw $390,000.
FAQ
How do I make a sales forecast in Excel?
Enter your last three monthly sales in cells A1, A2, A3 and months 1, 2, 3 in B1, B2, B3. The slope is simply =(A3-A1)/2, the intercept is =AVERAGE(A1:A3)-slope*2, and each future month k projects to =intercept+slope*k. Multiply by (1+seasonality/100) to add a peak or slowdown adjustment. This calculator does exactly that math for you in the browser.
How many months of history do I need?
Three months is the minimum this calculator accepts because two months cannot detect a curved or noisy trend. For a more robust forecast, six to twelve months of clean data is better, but you would want a weighted or seasonally-decomposed model (such as Prophet) rather than a straight line.
What is a realistic sales growth percentage for a small business?
Between 5% and 20% year-over-year is typical for a healthy small business; mature or declining businesses may see flat or negative growth. Compare your calculator's monthly trend to your trailing three-month average — if the slope implies more than 25% annualized growth without a clear driver, treat the forecast as optimistic.
Should I use linear trend or a percentage growth rate?
Linear trend adds a fixed dollar amount each month and works best for early-stage businesses with steady absolute growth. Percentage (compound) growth multiplies by a rate each month and works better once you have a stable base and predictable margins. This calculator uses linear because it is the more conservative and more commonly-taught baseline for small business planning.
How accurate is this forecast?
A three-point linear model explains only the recent trend; it does not account for new competitors, price changes, ad campaign shifts, or macroeconomic shocks. Treat the output as a planning baseline, not a promise. Track forecast error (the difference between projected and actual) each month and widen your range if the error is larger than 10–15%.
What does the seasonality adjustment actually do?
It multiplies every projected month by the same factor. A +20% entry raises each month's number by 20%; a −15% entry lowers it by 15%. It is intentionally flat across the horizon — if your peak is only in December, either run the calculator once for that month's window with the uplift, or split the year into seasonal blocks and forecast each block separately.
What if one of my three months was an outlier?
Replace that month with the average of the other two before entering the values, or run the forecast twice — once with the outlier and once without — and compare the range. A single anomalous contract (a big one-off B2B deal, a weather closure) will tilt a three-point line sharply and overstate your run-rate.
Disclaimer
This sales forecast calculator is provided for informational purposes only and does not constitute financial, accounting, or investment advice. The projection is a simple linear extrapolation of the three months you enter and is not a substitute for a rolling forecast prepared by a qualified accountant, market study, or historical seasonality analysis. Actual future sales depend on demand, competition, pricing, advertising, and many other factors outside this model. Always combine the output with your own judgment and up-to-date business data before making purchasing, hiring, or financing decisions.