Why you plan a budget backwards
A budget set forwards — "we can afford $10,000 a month" — tells you nothing about whether the campaign will work. A budget set backwards starts from the number the business actually needs and asks what has to be true for the media to deliver it. That reframing is the whole value of this calculator: it converts a vague affordability question into a list of specific, arguable assumptions.
The paid funnel is a chain of multiplications. Impressions become clicks at your click-through rate. Clicks become conversions at your landing page conversion rate. Conversions become revenue at your average order value. Run that chain forwards and you get a forecast; run it backwards and you get a requirement. Because every link is a multiplication, the requirement is a division at each step, and the arithmetic never gets harder than dividing by a decimal.
The output most people came for is the budget, but the output that most often changes a decision is the cost per acquisition compared with break-even. A plan can be perfectly executable — the impressions exist, the clicks are buyable — and still lose money on every sale. Seeing $150 of media against $60 of gross profit per order stops a campaign before it starts, which is far cheaper than discovering it in month two.
Note what the model deliberately excludes: creative production, agency fees, platform management time, and any lift the campaign gives to organic or direct traffic. It prices media only. Add the non-media costs separately when you present the number, or the total will be understated by whatever your agency charges.
The chain, one link at a time
Clicks required = conversions ÷ conversion rate. If you need 100 sales and 2% of clicks buy, you need 5,000 clicks. The rate must be measured click-to-conversion for the same traffic source. A site-wide conversion rate that mixes branded direct traffic with cold paid traffic will overstate paid performance, sometimes by several multiples, because branded visitors were already going to buy.
Budget = clicks required × cost per click. The click price is an auction outcome, not a constant, and it drifts with competition, quality score and match type. Use the last full month for the same campaign, and re-check it after any large budget increase — buying more volume usually means bidding on less efficient inventory. The cost per click calculator converts between CPC, CPM and click-through rate if your platform quotes you an impression price instead.
Impressions required = clicks required ÷ click-through rate. This link does not affect the budget on a click-priced buy — you pay per click regardless of how many impressions it took. It matters because it is a feasibility test: if the plan needs 4,000,000 impressions and the available inventory for your targeting is 1,000,000 a month, no budget makes the plan work and you need to widen the audience or lengthen the flight.
Break-even CPA = average order value × gross margin. This is the ceiling. Media is paid out of gross profit, not out of revenue, so a $120 order at a 60% margin can support at most $72 of acquisition cost before the sale contributes nothing. Comparing your planned CPA against this figure is the single most useful check in the model. If you acquire customers who buy repeatedly, the honest version of the ceiling uses customer lifetime value rather than a single order — but then you are funding acquisition out of future cash, which is a financing decision as much as a marketing one.
Worked example: 100 sales at a 2% conversion rate
You need 100 sales next month. Paid search converts at 2% of clicks, your average cost per click is $2.00, the campaign's click-through rate is 4%, your average order value is $100 and your gross margin is 100% because you sell a digital product with no unit cost.
- Clicks. 100 ÷ 0.02 = 5,000 clicks.
- Budget. 5,000 × $2.00 = $10,000.
- Impressions. 5,000 ÷ 0.04 = 125,000 impressions.
- Cost per acquisition. $10,000 ÷ 100 = $100. You can also get this directly: $2.00 ÷ 0.02 = $100, since CPA is always CPC ÷ conversion rate.
- Break-even CPA. $100 × 100% = $100.
- Verdict. Revenue is 100 × $100 = $10,000, so ROAS is $10,000 ÷ $10,000 = 1.0× and gross profit after media is $10,000 − $10,000 = $0. The plan exactly breaks even.
Now vary the assumption most likely to be wrong. If the conversion rate is 2.5% rather than 2%, clicks fall to 100 ÷ 0.025 = 4,000 and the budget falls to $8,000 — a 20% reduction from a half-point improvement, because 2.5 ÷ 2 = 1.25 and the budget scales with the reciprocal. If the rate is 1.5% instead, clicks rise to 6,667 and the budget rises to $13,333, which is 33% more than the plan. That asymmetry — the downside is larger than the upside for an equal-sized move in the rate — is the reciprocal at work, and it is the main reason to plan with a rate you have measured rather than one you hope for.
Reading the plan before you fund it
Start with cost per acquisition against break-even. If planned CPA sits above break-even, the campaign destroys gross profit on every sale and no amount of scale fixes it; the fixes are a higher conversion rate, a lower click price, a higher order value or a better margin, and you should name which one you are betting on. If planned CPA sits below break-even, the difference is the gross profit each conversion contributes towards fixed costs.
Then check ROAS against your own required threshold rather than against a generic target. The break-even ROAS is 1 ÷ gross margin: at a 60% margin you need 1 ÷ 0.6 = 1.67× just to cover the cost of goods and the media, and at a 25% margin you need 4.0×. A campaign running at 3× is excellent in the first business and loss-making in the second. The ROAS calculator works this threshold out for whatever margin you carry.
Then test feasibility on impressions. Compare the impression requirement against what the platform's forecasting tool says is available for your targeting and geography. A plan that needs more impressions than exist is not a budget problem — it is a targeting problem, and the answer is usually a second channel rather than a bigger bid.
Finally, treat every input as a range rather than a point. The sensitivity table on this page varies the conversion rate because that is the assumption with the widest error bars and the largest leverage, but you should run the same exercise on click price before a quarter-scale commitment. Present the budget as a band with the assumptions attached, and the conversation with finance becomes about which assumption to test first rather than about whether the number is trustworthy.
Budget needed for 100 conversions, by conversion rate and click price
| Conversion rate | Clicks needed | CPC $0.50 | CPC $1.00 | CPC $2.00 | CPC $5.00 |
|---|---|---|---|---|---|
| 0.5% | 20,000 | $10,000 | $20,000 | $40,000 | $100,000 |
| 1% | 10,000 | $5,000 | $10,000 | $20,000 | $50,000 |
| 2% | 5,000 | $2,500 | $5,000 | $10,000 | $25,000 |
| 3% | 3,333 | $1,667 | $3,333 | $6,667 | $16,667 |
| 5% | 2,000 | $1,000 | $2,000 | $4,000 | $10,000 |
| 10% | 1,000 | $500 | $1,000 | $2,000 | $5,000 |
Cost per acquisition is the budget divided by 100 in every cell — for example $10,000 ÷ 100 = $100 at 2% and $2.00 a click, which equals CPC ÷ conversion rate.
Assumptions that break this model
- Using a site-wide conversion rate for paid traffic. Branded and direct visitors convert far better than cold paid clicks. Segment the rate by source or the budget will be understated.
- Assuming the click price holds as you scale. Extra budget buys progressively less efficient inventory. A large increase usually raises average CPC, so re-forecast after each material step up.
- Pricing media out of revenue instead of gross profit. Break-even CPA is order value × margin. Comparing CPA to order value alone will approve campaigns that lose money on every sale.
- Forgetting the non-media costs. Creative, agency retainer, landing page work and tooling sit outside this budget. Add them before presenting a total, or your true cost per acquisition is higher than the model says.
- Ignoring the lag between click and conversion. If your sales cycle is 30 days, a month's spend produces conversions that land in the following month. The budget is right; the timing of the return is not.
- Treating the impression figure as a cost. On a click-priced buy, impressions are a feasibility check only. On an impression-priced buy the budget is impressions ÷ 1,000 × CPM instead, which is a different calculation.
Where this sits among the other planning tools
This calculator prices a channel against a conversion goal. Two adjacent questions need different tools. If your goal is revenue rather than conversions and you want to know how much traffic — paid or otherwise — is required, use the traffic needed for revenue goal calculator, which starts from the revenue number and works through order value first. If you already have spend and results and want to grade what happened rather than plan what should, use marketing ROI or cost per acquisition.
Platform forecasting tools answer a narrower question well: Google's Performance Planner and the equivalent tools in other platforms model the auction, so they can tell you how conversions respond to bid and budget changes on inventory they can see. What they cannot do is apply your gross margin, your non-media costs or your repeat-purchase economics. Use the platform for the auction and this model for the business case; they are complements, not substitutes.
Once a campaign is live, the assumption to update first is always the conversion rate, because it is the one you measure fastest and the one with the largest leverage on budget. Update the click price second and the click-through rate last, since the last of the three does not move the money on a click-priced buy at all. For a subscription business, replace order value with first-period revenue and check the result against the CAC payback period calculator, because the relevant question there is not whether the first order pays for the click but how many months of margin it takes to get the cash back.