Short answer: How do you calculate an ad budget? You divide the revenue goal by average order value to get the number of orders you need. Dividing orders by the site conversion rate gives the number of clicks you need. Multiplying clicks by the average cost per click gives your monthly ad budget. For example, a 100,000 USD revenue goal, a 100 USD order value, a 2% conversion rate and a 0.48 USD CPC call for a 24,000 USD budget. Rebel Co. Group built this calculator so you can set a budget backward from a goal instead of guessing.
Take average order value from your e-commerce panel or from the average purchase revenue in the Monetization report in GA4. Read the site conversion rate in the Traffic acquisition report in GA4, filtered to paid traffic only if you can, because ad visitors convert lower than the overall average. For cost per click, enter the Avg. CPC column on the Campaigns screen in Google Ads if you have a running account, or Keyword Planner estimates if you do not. For Meta, use the cost per link click value in Ads Manager.
The calculator works with the values you enter. Your data stays in your browser and is never sent to a server. Results support decisions and do not replace accounting records.
Formula
The ad budget is calculated backward from the revenue goal in four steps, and each step builds on the previous result.
- Orders needed = Monthly revenue goal / Average order value
- Clicks needed = Orders needed / Site conversion rate
- Monthly budget needed = Clicks needed x Average cost per click (CPC)
- Implied ROAS = Revenue goal / Budget needed
- CAC = Budget needed / Orders needed, which is CPC / Conversion rate
The strength of the formula is that it shows the three levers that set the budget at the same time. Double the conversion rate and the budget is cut in half, cut CPC by 20% and the budget falls 20%, raise order value and you reach the same revenue with fewer orders. The budget is a result, not an input.
If you use more than one channel, do the calculation separately for each channel. Google search, Meta and Performance Max work with different CPCs and different conversion rates, and a budget calculated with a single average fits none of them exactly. Set the revenue share for each channel, calculate its budget with that channel's own CPC and conversion rate, then add them up. The total usually differs from the figure you get with a single average, and that gap points to the first correction in your channel plan.
Worked example
Let us calculate a brand that targets 100,000 USD of monthly revenue, with an average order value of 100 USD, a 2% paid traffic conversion rate and a 0.48 USD average cost per click.
| Step | Calculation | Result |
|---|---|---|
| Orders needed | 100,000 / 100 | 1,000 orders |
| Clicks needed | 1,000 / 0.02 | 50,000 clicks |
| Monthly budget needed | 50,000 x 0.48 | 24,000 USD |
| Implied ROAS | 100,000 / 24,000 | 4.17x |
| CAC | 24,000 / 1,000 | 24 USD |
If the conversion rate rose from 2% to 2.5%, the budget needed would fall to 19,200 USD. A half-point improvement means 4,800 USD in savings.
Is this budget profitable? The answer depends on your margin. With a 30% gross profit margin, break-even ROAS is 3.33x and an implied ROAS of 4.17x is profitable. With a 20% margin, break-even is 5x and this plan loses money on every order. Before you approve the budget, test the implied ROAS against your margin in the ROAS calculator.
How to read the result
The budget the calculator gives is a minimum. In real campaigns the learning period and competitive swings call for an extra 15-25% on top.
Read three numbers together. If the implied ROAS is above your break-even, the plan is realistic. If CAC is smaller than your contribution margin per order, every order leaves a profit. The number of clicks needed must fit within the monthly search volume of your target keywords, otherwise you have a budget but no traffic. According to industry reports, the median Google Ads ROAS in e-commerce is about 3.3x, and around 5x for shopping campaigns. If your implied ROAS comes out at 6x or higher, check your inputs, because most often the conversion rate was entered too optimistically.
The click volume test is the step most plans skip. For the 50,000 clicks in the example, the total monthly search volume of your target keywords needs to be at least 500,000, because the click-through rate in the search network runs at about 5-10%. Check the total volume of your target keyword group in Keyword Planner. If the volume is not enough, you have three options: widen the keyword set, open up to discovery channels such as Performance Max and Meta, or bring the revenue goal down to a realistic level. If you have a budget but no traffic, the calculated figure can never be spent and the goal stays on paper.
The budget question is not "how much should we spend" but "how many clicks does it take to reach this goal". The answer comes from multiplication, not from guessing.
Common mistakes
- Applying the site's overall conversion rate to ad traffic. The overall rate includes loyal customers and organic traffic. Paid traffic is new visitors, and on most sites it converts at about half the overall average. Filter by channel in GA4.
- Treating the Keyword Planner CPC as real. The planner gives an estimate, and the real cost changes with quality score and competitor density. If you have a running account, use the Avg. CPC of the last 30 days.
- Ignoring the season. During Black Friday and the year-end holidays, CPC rises noticeably. Planning a November budget with March data misses the goal.
- Including organic revenue in the ad goal. The revenue goal should be only the revenue you expect from ads. If you enter the total revenue goal, the budget comes out larger than it should.
- Opening the whole budget on day one. Campaigns learn in the first 1-2 weeks, and during that period CPC is high and conversion is low. Raise the budget gradually.
How to improve this number
- Raise the conversion rate. This is the lever that moves the budget most. For the product page, checkout step and trust signals, see our CRO guide.
- Lower CPC. Raise quality score, update your negative keyword list weekly and clean up the search terms report. The details are in the Google Ads costs guide.
- Increase average order value. The same revenue with fewer orders means fewer clicks and a smaller budget. The methods are in this article.
- Set up remarketing. Campaigns that go back to people who visited the site and did not buy convert several times higher than cold traffic and pull the average conversion rate up.
- Move budget to the campaign that brings profit. Track the ROAS of each campaign separately and stop the ones below break-even. The guide on using your ad budget efficiently builds this discipline.
- If you are starting from zero, test small. In the first month, measure the real CPC and conversion rate with 30-40% of the calculated budget, then run the calculator again with real data. The second calculation, made with measured inputs instead of estimates, is often markedly different from the first.
Frequently asked questions
What should the minimum budget for Google Ads be?
There is no technical minimum, but you need at least 300-500 clicks a month for meaningful data. At a CPC of 0.40 USD that is 120-200 USD, and a realistic starting point for e-commerce in most industries is 2,400-4,000 USD a month. With smaller budgets the algorithm cannot learn and cost per conversion stays high. Derive your budget from the clicks and conversions you need, not from the goal alone.
What percentage of revenue should the ad budget be?
A common rule is 10-20% of revenue for new brands and 5-10% for established ones. This ratio changes with industry and margin, though. The right approach is to work from break-even ROAS instead of a percentage rule: you can raise the budget as long as the implied ROAS stays above your break-even. Use the percentage rule only for a first draft, and let contribution margin and implied ROAS make the decision. Spending 20% of revenue on a thin-margin product loses money, while spending 30% on a wide-margin product can make money.
What should I enter if I do not know my conversion rate?
In GA4, you can find the purchase rate per session in the Monetization report. If you have no data at all, industry reports say e-commerce sites typically convert at 1-3%, and for paid traffic that band drops to about 0.8-2%. Start conservatively, enter 1.5%, and update it with the real data from the first month. An optimistic conversion rate makes the budget look small and the goal unreachable. If possible, use the rate for paid traffic only, because the overall site rate is always higher.
Is a Meta Ads budget calculated the same way?
Yes, the formula is the same but the inputs behave differently. On Meta the cost per click is usually lower than in Google search, but the conversion rate is lower too, because users arrive in discovery mode instead of with search intent. For Meta, enter the cost per link click from Ads Manager and a conversion rate measured for Meta traffic only. Calculate the two channels separately and add them up, rather than using one calculation with averages.
How do I know if the calculated budget is profitable?
Compare the implied ROAS with break-even ROAS. Break-even ROAS is 1 divided by gross profit margin. If your margin is 40%, break-even is 2.5x, and if the calculator gives 4x, the plan is profitable. Also compare CAC with contribution margin per order, because if CAC is larger than the margin, every order loses money. You can run these two tests in a few minutes with the ROAS calculator and the e-commerce profit calculator.