Forecasting expenses means estimating your future costs by combining what you already spend with what you expect to change. Whether you’re building a budget for a small business, preparing projections for investors, or planning a department’s spending for next year, the process follows the same core steps: categorize your costs, choose a forecasting method, adjust for real-world factors like inflation and seasonality, and build in a buffer for surprises.
Separate Fixed, Variable, and Semi-Variable Costs
Before you can project anything forward, you need to understand how each expense behaves. Costs fall into three categories, and each one gets forecasted differently.
Fixed costs stay the same regardless of how much you produce or sell. Rent, property taxes, insurance premiums, loan interest payments, and depreciation on equipment are all fixed. These are the easiest to forecast because they’re predictable. If your lease is $4,000 a month this year and the contract doesn’t change, it’s $4,000 a month next year.
Variable costs rise and fall with your business activity. Raw materials, hourly labor, sales commissions, shipping, and many utility bills fall here. If you expect to sell 20% more product next quarter, your variable costs will climb roughly in proportion. To forecast these, you need a sales or production forecast first, then tie each variable cost to that volume.
Semi-variable costs (sometimes called mixed costs) combine both elements. They stay flat up to a certain level of activity, then start climbing. A phone plan with a base fee plus overage charges is a simple example. A warehouse team that works regular hours until orders spike, triggering overtime pay, is a bigger one. When forecasting these, estimate the fixed portion separately from the variable portion so you don’t overcount or undercount as volume shifts.
Pull at least 12 months of actual spending data and tag every line item as fixed, variable, or semi-variable. This categorization is the foundation everything else builds on.
Choose a Forecasting Method
Two approaches dominate expense forecasting, and they suit different situations.
Incremental Forecasting
This is the most common method. You take last year’s actual spending (or this year’s budget) as your starting point, then adjust each line item for expected changes: price increases from suppliers, a new hire, a software subscription you’re adding. It’s fast, straightforward, and works well when your business is stable and your cost structure isn’t changing dramatically.
The downside is that incremental forecasting assumes everything you spent money on last year still makes sense. It carries forward inefficiencies without questioning them. Departments that know unspent budget will be cut next year tend to spend just to protect their allocation, which inflates the baseline you’re building from.
Zero-Based Forecasting
Zero-based forecasting starts from scratch. Instead of adjusting last year’s numbers, you justify every expense as if you were building the budget for the first time. Each activity gets a cost estimate tied to a specific objective or output. Only after every line item has been evaluated and approved does the full forecast come together.
This method forces transparency and tends to surface costs that have been running on autopilot. It’s especially useful when you’re restructuring, entering a new market, or trying to cut spending significantly. The tradeoff is time. Zero-based forecasting takes considerably more effort and can create friction between departments competing for resources.
Many businesses use a hybrid: incremental forecasting for routine, stable cost categories and zero-based reviews for areas where spending has grown or where priorities are shifting.
Build Your Projections Step by Step
Once you’ve categorized your costs and picked a method, the actual forecasting process looks like this:
- Gather historical data. Pull your actual spending for the past two to three years, broken out monthly. Annual totals hide important patterns. You want to see how costs moved month to month, not just in aggregate.
- Lock in known fixed costs. Enter contract amounts for rent, insurance, loan payments, and any other costs you can confirm from existing agreements. These need the least guesswork.
- Link variable costs to your revenue or production forecast. Calculate the ratio of each variable cost to your sales volume (cost per unit sold, commission as a percentage of revenue, shipping cost per order) using your historical data. Then multiply those ratios by your projected volume for the forecast period.
- Estimate semi-variable costs in two pieces. Forecast the fixed base, then add the variable portion tied to expected activity levels. If your delivery fleet costs $3,000 a month in base expenses plus $0.40 per mile, forecast the base and the mileage separately.
- Add planned new expenses. Any hires, equipment purchases, software subscriptions, or marketing campaigns you’re planning should be entered as specific line items with start dates, not lumped into a general increase.
Adjust for Inflation and Wage Growth
Even if nothing changes about your operations, your costs will rise simply because prices do. Skipping an inflation adjustment is one of the fastest ways to end up with a forecast that’s too low by year’s end.
For general business costs, the Congressional Budget Office projects consumer price inflation of around 2.7% in 2026. That’s a reasonable baseline for supplies, services, and other everyday expenses. But not all costs move at the same rate. The CBO projects private-sector wages and salaries to grow by 3.5% in 2026, which means your labor costs will likely outpace your other expenses. If payroll is your biggest expense category (it is for most service businesses), applying a blanket 3% increase across the board will undercount your actual spending.
Apply inflation adjustments at the category level. Use wage growth rates for payroll and benefits. Use general inflation for supplies and services. And check your specific contracts: if a supplier has already quoted a 5% increase for next year, use that number instead of an average.
Account for Seasonal Swings
Monthly expenses rarely follow a flat line. A retail business spends more on inventory and temporary staff in the fall. A landscaping company’s fuel and labor costs spike in summer and drop in winter. HVAC expenses climb in the hottest and coldest months for almost any business with a physical location.
The most practical way to handle this is to look at your historical monthly spending and calculate each month’s share of the annual total for each cost category. If your shipping costs over the past two years averaged 12% of the annual total in November but only 5% in February, use those same proportions to distribute your forecasted annual shipping costs across the months ahead.
This approach, sometimes called a seasonal index, works well for recurring patterns. Where it breaks down is when something unusual distorts your historical data, like a one-time equipment failure that spiked repair costs in March or a pandemic that crushed travel spending for a full year. When you spot those anomalies, adjust the affected months before calculating your seasonal ratios so the outlier doesn’t skew your forecast.
Add a Contingency Buffer
No forecast captures everything. Equipment breaks. A key supplier raises prices mid-year. A new regulation adds a compliance cost you didn’t anticipate. Building a contingency line item into your forecast gives you room to absorb these without blowing your budget.
A common rule of thumb is to set aside 10% to 15% of your total forecasted expenses as contingency. The exact percentage depends on how predictable your cost environment is. A business with long-term contracts and stable operations might be comfortable at 10%. A company entering a new market or managing a large construction project might need closer to 15% or even higher.
Keep the contingency as a separate line item rather than padding individual categories. This makes it visible, trackable, and harder to spend casually. If you sprinkle extra dollars across every line, those cushions tend to get absorbed into routine spending without anyone noticing.
Review and Update Regularly
A forecast created in January and never revisited will be meaningfully wrong by June. The value of forecasting isn’t in producing a single document; it’s in the ongoing comparison between what you projected and what actually happened.
Set a monthly or quarterly cadence to compare actual expenses against your forecast. When a category is consistently running over or under, investigate why and update the remaining months. This rolling approach turns your forecast into a living tool rather than a static spreadsheet. It also sharpens your future forecasts because you’ll learn where your assumptions tend to be off and by how much.
Track your variance in both dollars and percentages. A $500 overage on a $2,000 line item (25% over) deserves more attention than a $500 overage on a $50,000 line item (1% over), even though the dollar amount is the same. Over a few cycles, you’ll develop a sense for which categories are reliably predictable and which ones need wider ranges in your projections.