A fixed-cost change moves profit by pulling two cost behaviors against each other. Fixed costs stay put as work rises or falls, while variable costs move with the volume of work. In one worked example, a 10% rise in fixed costs paired with a 5% cut in variable costs lifts net operating income by 35%. Whether a change like that helps your bid depends on how those two effects net out.
Key takeaways
- Sort every cost line in the bid into fixed or variable before you change any number.
- Write the fixed-cost move and the variable-cost move on separate lines, and hold sales steady for the first test.
- Compare the two versions line by line, from sales down to net operating income.
- Lower sales in small steps to find the point where the gain disappears.
Every bid carries two kinds of cost. Some costs move with the work, and some stay the same no matter how much work you win. When an owner or estimator changes how a job is built, say by buying equipment that replaces some hourly labor, the mix shifts, and profit shifts with it. The hard question is how much.
This post answers that question with one worked example from a managerial accounting textbook, in its section on margin of safety and operating leverage. The example is about a store, not a construction bid or a service quote, but the arithmetic is the same for any business that splits its costs this way.
Three percentages carry the whole test: a 10% rise in fixed costs, a 5% drop in variable costs, and the 35% increase in net operating income that the two moves produce together. Once you can follow those three numbers, you can run the same test on your own bid.
1. Sort every bid cost into fixed or variable
Start with the cost lines in the bid, and mark each one as fixed or variable. A variable cost rises and falls with the volume of work you deliver.
A fixed cost stays about the same across a normal range of volume. Insurance premiums, equipment leases, and the rent on a yard are common examples.
Some lines are mixed, meaning part of the cost is fixed and part moves with volume. You have to split those by hand, and the split is a judgment you should write down. Do it on paper before you change anything. Every later step depends on it.
Past job records are the most useful source for the split. Timesheets show how many hours a crew worked on each kind of job. Equipment statements show costs that were the same whether one job or five ran that month.
For a mixed line such as utilities or a shared vehicle, compare what the line cost in a busy period with what it cost in a slow one. The difference gives a rough sense of how much of the line moves with volume and how much stays put. Note the method beside each mixed line, so that anyone reviewing the bid can see how the split was made.
The OpenStax example starts from a clear split. Its store has sales of $125,000,000 per year, and the figures in its comparison are shown in thousands. Before the change, sales are 125,000 thousand, variable costs are 93,750 thousand, and fixed costs are 25,000 thousand. Those three numbers are the whole starting point.
Two derived lines make the split useful. Contribution margin is sales minus variable costs, and it is the money left over to cover fixed costs. Net operating income is contribution margin minus fixed costs. For the store, 125,000 minus 93,750 leaves 31,250 of contribution margin, and 31,250 minus 25,000 leaves 6,250 of net operating income.
Write these five lines on a small sheet: sales, variable costs, contribution margin, fixed costs, and net operating income. Keep the same layout for every scenario you test, so each one is built the same way and the results compare line by line. A sheet that changes shape from one version to the next is where errors usually hide.
2. Write the change as two separate moves
The store in the example is weighing a decision with two effects. The OpenStax example describes the change as one that "increases its fixed costs by 10% but reduces its variable costs by 5%." That sentence is the core of the whole test, and it is easy to misread when it gets compressed into a single profit estimate.
Write each move on its own line. The fixed-cost move is plus 10%, and it applies only to the fixed-cost line. The variable-cost move is minus 5%, and it applies only to the variable-cost line.
Do not apply either percentage to total cost. A 10% increase to a total that mixes fixed and variable costs blends two different behaviors into one number and hides which one drives the result. The two percentages also sit on different bases, so a single blended figure cannot show which side matters more.
The two moves push profit in opposite directions. The higher fixed cost is a charge that does not shrink when work is slow, so it comes off the top every period. The lower variable cost is a saving on every unit of work, so its benefit grows with volume. Whether the change helps depends on whether the saving on the variable side is larger than the added charge on the fixed side. The rest of this post shows how to check that, one line at a time.
Before you build the second version of the sheet, set the sales line to its current value and leave it there. Sales should not move in this first test. If the change also brings in new work, that is a second scenario, and it should be tested separately so the cost effect stays readable.
A useful habit is to write the two moves as plain sentences before you touch the numbers. A sentence such as "fixed costs go up by 10 percent, and variable costs go down by 5 percent" forces you to name the direction of each move. Sign mistakes, where a cost that should fall is entered as a rise, are easy to make on a busy sheet. The sentence doubles as a check on what you enter.
Illustrative example: one small bid
Suppose a bid has sales of 50, variable costs of 35 and fixed costs of 10. Contribution margin is 15, and net operating income is 5. The two moves net out to zero, so the test is whether the variable saving is larger than the added fixed charge.
3. Compare the two scenarios side by side
Build both versions of the sheet, one without the change and one with it. The OpenStax example places them in a single comparison, and the figures are reproduced here as the example gives them. All amounts are in thousands.
Sales do not change, so the whole difference comes from costs. Fixed costs rise from 25,000 to 27,500, which takes back part of that gain. What remains is net operating income of 8,438, up from 6,250. The comparison in the linked textbook section shows the same values. The printed contribution margin is off by 1 thousand from sales minus variable costs, which points to rounding in the source, so check your own sheet against its own totals.
Read the table from top to bottom rather than jumping to the last row. A reader who sees only the final profit line cannot tell whether the gain came from cheaper variable work or from somewhere else. The contribution margin row shows where the saving landed, and the fixed-cost row shows what the change costs to keep running in every period. With both rows visible, a reviewer can ask the right question: is the saving on the variable side durable, and will the fixed cost still be carried if the work slows?
Keep the sheet in this exact shape for every scenario you add. If you test a third version, add a column rather than rebuilding the table, so the first two columns stay fixed for comparison. That habit is the simplest protection against a bid that looks good only because its inputs changed between versions.
4. Read the 35% gain in dollars as well as percent
The OpenStax example reports that, at the current sales level, the change "could produce a whopping 35% increase in net operating income." The percentage checks out. Net operating income moves from 6,250 thousand to 8,438 thousand, which is a 35 percent increase.
A percentage gain on a modest base can look bigger than it is. A percentage on a large base can look small even when the dollars matter. Before you decide, look at the dollar figures next to the percent.
Net operating income rises in dollars, and that is the number that pays for the crew, the insurance, and the owner's time. The fixed-cost line also rises, from 25,000 thousand to 27,500 thousand. That increase is a commitment that stays in place even if the work does not arrive.
Notice where the saving came from. The variable-cost cut is only 5 percent, but it applies to a large base: variable costs are 93,750 thousand before the change and 89,063 thousand after it. The fixed-cost rise is 10 percent, applied to a smaller base of 25,000 thousand. Comparing 10 with 5 tells you very little on its own. Compare the dollars on each line, and let the sheet show which side is larger.
The 35 percent figure also belongs to one sales level. The example calls it the result "at the current sales level," and that qualifier matters. If volume rises or falls, the percentage changes too. The 35 percent answers one question, which is what happens at today's volume. Your bid needs to answer a second question, which is what happens when volume is different. That is the subject of the next step.
This sensitivity has a name in managerial accounting: operating leverage. A business with a larger share of fixed costs sees a bigger swing in profit from each change in sales, in either direction, because the fixed costs stay put while sales move. The linked textbook section covers the measure in more detail.
For a bid, the practical version is simple. The more of the cost base that is fixed, the more the profit line reacts to volume. A change that raises fixed costs and cuts variable costs therefore changes the bid's exposure to volume as well as its expected profit. The sales test in the next step is how you see that exposure.
5. Find how far sales can fall before the gain disappears
A change that improves profit at today's volume can still be a poor bet if the volume drops. The OpenStax example runs this test directly: "If customers disliked the change enough that sales decreased by more than 6%, net operating income would drop below the original level of $6,250 and could even become a loss." In the store's table, that original level is 6,250 thousand.
That sentence holds three things an estimator should copy into any bid test. The first is the threshold. A fall in sales of more than 6 percent is the point where the new setup stops beating the old one. The second is the floor. The third is the downside. A large enough fall in sales can push profit below the original level and even into a loss, so the test is about more than whether the change looks good on paper.
To run the same test on your own bid, take the new scenario and lower sales in small, equal steps. At each step, recompute variable costs at the lower volume, then contribution margin, then net operating income. Hold fixed costs at their new level, because they are the part of the bid that does not fall when work slows. Stop when net operating income reaches the original figure or falls below it. The sales level where that happens is the cushion your bid has.
Run the same steps in the other direction as well. If sales rise, the variable-cost saving applies to more units, and the new setup pulls ahead of the old one by a wider margin. A higher-volume case shows how much the bid gains from the change if the work comes in strong. Showing both directions gives the owner a range instead of a single hopeful figure, and a range holds up better when the job is finished and the actual results come in.
If your expected volume sits close to the threshold, the change is fragile, even if the headline percentage looks attractive. A bid that depends on a narrow cushion is exposed to a single slow season, a lost contract, or a customer who decides the new process is not worth the price. Put the cushion in writing next to the 35 percent figure, so that the owner or client sees both at the same time.
If the cushion is thin, you still have options that do not require abandoning the change. You can phase the change in over several jobs and watch the actual split before committing the full fixed cost. You can also ask whether the new fixed charge can be tied to a shorter contract term, which limits how long the commitment runs if volume disappoints. Each option changes the sheet, so rerun the sales steps after you make it.
Be careful with the word "cushion," though. In the OpenStax example, the cushion is a sales decline of more than 6 percent. That number applies to the store's cost structure at its current sales. It does not carry over to your bid unless your cost split matches the store's. Run the steps on your own sheet, and let your numbers set the threshold. For more on this kind of check, read margin of safety and bid sensitivity tests.
6. Check each scenario before the bid goes out
Once the sheet is built, run each scenario through a calculator so the arithmetic cannot drift between versions.
A calculator solves for whatever result you give it, so the quality of the test depends on the split you feed in. If the fixed and variable lines are wrong on the sheet, the calculator will produce a clean, precise, and wrong answer. Check the inputs against the bid, and check the split against invoices and payroll records, before you trust any output.
Before the bid leaves your desk, run the sheet through the checks in this table:
If any check fails, fix the sheet before you read the profit line again. A weak version can look finished when its profit line is high.
When you present the result, show the two versions side by side, the dollar change on each line, the percentage, and the sales level at which the gain disappears. That layout answers the questions an owner is most likely to ask without a long explanation. Keep the sheet and the calculator output together in the bid file, so the numbers can be rechecked later when actual costs are known.
Many cost systems report a single unit cost, and that figure can hide the split you worked hard to build. The Xero guide to absorption costing states that "absorption costing assigns all manufacturing costs, both fixed and variable, to each unit you produce." A unit cost built that way is useful for pricing, but it blends the two behaviors into one figure, and it can hide how profit moves when only the fixed side changes.
Run the three percentages as a set. A 10 percent fixed-cost rise, a 5 percent variable-cost cut, and a 35 percent profit gain tell a story only when you read them with the dollar rows and the sales threshold. Rerun the sheet whenever a cost line changes, and let the numbers decide before the bid goes out.
Next step: today, pick one bid you have already priced, sort its cost lines into fixed and variable, and fill in the five-line sheet from step 1 before you change any number.
Related guides: Late freight: 4 steps to update posted material cost · Configure 8 profit settings in your estimating tools.
Frequently asked questions
Does a higher fixed cost always cut profit?
No, a higher fixed cost does not always cut profit: in the worked example, a 10% fixed-cost rise came with a 5% variable-cost cut, and net operating income still rose 35%. Whether your own bid comes out the same depends on your cost split.
How far can sales fall before the gain disappears?
In the OpenStax store example, the gain disappears once sales fall by more than 6%. Run the steps in section 5 on your own sheet, because your threshold depends on your cost split.
What separates a fixed cost from a variable cost?
A variable cost rises and falls with the volume of work you deliver, while a fixed cost stays about the same across a normal range of volume.