How much should I be spending on ads next month?

Why your ad budget should be driven by margin math, not gut feel

You spent $8,000 on ads last month. Google says you got $32,000 in conversions. That looks like a 4x return. But Google counts every conversion, including the ones who never paid, the ones who returned the product, and the ones who would have bought anyway.

When you check your actual accounting data, $8,000 in ad spend generated about $22,000 in collected revenue. Your gross margin on that revenue is 55%, so you kept $12,100 in gross profit. After the $8,000 in ad spend, you netted $4,100. That is still profitable, but it is a 1.5x real return, not 4x.

Now the question becomes: if you spend $12,000 next month instead of $8,000, does the return hold? Or does it decline at higher spend levels? And can your cash reserves handle the higher outlay while you wait for that revenue to actually arrive?

The three inputs to a data-driven ad budget

• Your gross margin. Revenue minus cost of goods sold, divided by revenue. This tells you how much of each ad-generated dollar you actually keep before overhead. If your gross margin is 50%, every $100 in ad-driven revenue gives you $50 to cover ad costs and still profit.
• Your real return on ad spend (ROAS). Not what Google or Meta reports. Your actual ROAS: total collected revenue from ad-sourced customers divided by total ad spend. This requires matching your ad platform data to your accounting data.
• Your available cash for growth spending. How much cash can you deploy on ads without creating a cash crunch? This is your current cash minus committed expenses for the next 30-60 days, minus a safety buffer.

How to calculate your ad budget using QuickBooks and your ad platforms

1. 1
   Get your gross margin from the P&L

   Go to Reports→ “Profit and Loss.” Set it to last month. Find Total Income and Cost of Goods Sold. Gross margin = (Total Income - COGS) / Total Income.

2. 2
   Find your total ad spend

   In the P&L, look at your Advertising or Marketing expense line. Alternatively, log into Google Ads, Meta Ads Manager, or your other platforms and add up total spend for last month.

3. 3
   Estimate ad-sourced revenue

   This is the hardest step. Check your ad platform conversion data and cross-reference it with actual payments in QuickBooks. If you cannot match precisely, use a conservative estimate: take the platform-reported revenue and discount it by 30-40%.

4. 4
   Calculate your real ROAS

   Real ROAS = ad-sourced collected revenue / ad spend. If you spent $8,000 and collected $22,000 from ad-sourced customers, your real ROAS is 2.75x.

5. 5
   Check your cash availability

   Go to Reports→ “Balance Sheet.” Note your Total Bank Accounts. Subtract your upcoming obligations for the next 60 days. What remains is your growth budget ceiling.

6. 6
   Set the budget

   Your profitable ad budget = the amount where (ad spend x real ROAS x gross margin) still exceeds ad spend. Your safe ad budget = the lower of that number and your available cash for growth. Start there.

How to calculate your ad budget using Xero and your ad platforms

Same logic, slightly different navigation. Xero calculates gross profit for you in the P&L, which saves one step.

1. 1
   Get gross margin from the P&L

   Go to Accounting → Reports→ “Profit and Loss.” Xero shows Gross Profit directly. Divide Gross Profit by Total Revenue for your gross margin percentage.

2. 2
   Find your ad spend in operating expenses

   Look under Operating Expenses for your advertising or marketing line items. Cross-reference with your ad platform dashboards.

3. 3
   Match ad conversions to actual payments

   Same challenge as QuickBooks. Compare what the ad platforms report as conversions to what actually showed up as paid invoices in Xero. Discount platform numbers by 30-40% if you cannot match exactly.

4. 4
   Check cash and set the budget

   Go to Accounting → Reports→ “Balance Sheet.” Check Bank under Current Assets. Apply the same formula: profitable budget capped by available cash.

What it takes to recalculate your ad budget every month

• ROAS changes month to month.Seasonality, competition, and ad fatigue all affect return. Last month's ROAS is not a guarantee of next month's.
• Matching ad data to revenue is tedious. You are cross-referencing Google Ads, Meta, maybe TikTok, against your actual accounting records. Every month.
• Cash position fluctuates. A great ad budget in March might be reckless in April if a big client is late on payment.

Or get a data-driven ad budget recommendation every month

Bottomline connects to your accounting software and your ad platforms. It matches ad spend to actual collected revenue (not platform-reported conversions), calculates your real ROAS, and factors in your cash position to recommend a budget:

No more guessing. No more trusting what Google tells you. Bottomline uses your real accounting numbers to tell you what you can afford to spend and what you should expect to get back.