Budget pacing formula, with the 30.4 rule
Expected spend = monthly budget / 30.4 × days. Variance = (actual - expected) / expected. A worked euro example, plus the 31-day and February skew.
The budget pacing formula is expected spend = monthly budget / 30.4 × days, and variance % = (actual - expected) / expected × 100. A €4,560 monthly budget gives €150 a day, so €1,800 is expected after 12 days. The 30.4 is the multiplier Google Ads uses for its monthly spending limit.
daily budget = monthly budget / 30.4
expected spend = daily budget * days in the window
variance % = (actual spend - expected spend) / expected spend * 100
A positive variance means spending ahead of plan, a negative one behind it. Underspend is a miss too, because unspent budget buys your client nothing.
Where does 30.4 come from?
Google Ads Help sets two spending limits tied to your average daily budget. Both sentences below are quoted from Google Ads Help, answer 6385083, checked 3 September 2026:
Your daily spending limit (two times your average daily budget for most campaigns) on any particular day.
Your monthly spending limit (30.4 times your average daily budget for most campaigns) in any particular month.
365 days divided by 12 months is 30.42. Rounded to one decimal, that is 30.4. It is the length of an average month, not of any real one. The constant has its own glossary entry, the synthetic 30.4-day month.
Read the monthly limit backwards and you have the pacing formula. Divide a monthly budget by 30.4 and you get the daily budget whose monthly limit equals that budget, for most campaigns. A €4,560 monthly budget gives a €150 daily budget, and 30.4 × €150 is €4,560 again. The Google Ads daily budget calculator does that division for you.
The daily limit changes how you read pacing. At €150 a day, Google may spend up to €300 on any particular day. One high day sits inside the published rule, so judge pacing over a window of days, never a single day.
The formula itself is not a platform rule. It is arithmetic on your monthly number, so you can pace a Meta budget with it too.
A worked example in euros
Take a client on €4,560 a month. It is 12 October, so 12 days of the month have passed, today included. Google Ads and Meta together report €1,980 of spend.
monthly budget €4,560
daily budget 4,560 / 30.4 = €150.00
expected, 12 days 150 * 12 = €1,800.00
actual Google Ads + Meta = €1,980.00
variance (1,980 - 1,800) / 1,800 = +10.0%
The client is 10% ahead of plan. That is €180 more than the first 12 days allowed. You can recompute every line by hand, or enter the three inputs in the budget pacing calculator.
October has 31 days, and that changes how you should read the 10%.
Why does an even spender show -1.9% in a 31-day month and +8.6% in February?
30.4 is never the length of the month you are in. Picture a client who spends the budget evenly across the calendar month. Each day gets the budget divided by the real number of days, while the formula expects the budget divided by 30.4.
In a 31-day month, expected spend runs about 2.0% above an even calendar split, because 31 / 30.4 = 1.020. The even spender therefore shows about -1.9%. That reads as a mild underspend for a client who will land exactly on budget.
In a 28-day February, expected spend runs about 7.9% below an even split, because 28 / 30.4 = 0.921. The same even spender shows about +8.6%. On any tolerance of 5% or less, that reads as overspend.
days in month expected vs even split even spender shows
31 +2.0% -1.9%
30 -1.3% +1.3%
29 -4.6% +4.8%
28 -7.9% +8.6%
With the €4,560 budget, an even spender in October spends €147.10 a day. Over 14 days that is €2,059.35 against €2,100.00 expected, a variance of -1.9%. In a 28-day February the daily spend is €162.86. Fourteen days make €2,280.00 against the same €2,100.00, a variance of +8.6%.
So read each month against its own baseline. In February, +8.6% is on plan and +2% is about 6% behind an even calendar split. In the October example, +10.0% is about 12% ahead of one.
Month-to-date or rolling window?
The formula needs a number of days, and there are two ways to choose them.
Month-to-date counts from the 1st to today. It answers the question your client asks: will this month land on budget? It resets on the 1st, so early readings rest on very few days. On the 2nd, a €300 day followed by a €150 day puts the client at +50%. That is €450 spent against €300 expected.
A rolling window counts the last N days ending today, such as 14 or 30. It does not reset on the 1st, so a 30-day window on the 3rd still rests on 30 days of spend. It shows the current run rate. The cost is that it reaches back into last month, and those older days are then judged by this month's budget.
Use both if you can. Month-to-date tells you where the invoice will land. A 14 or 30-day window tells you whether the current run rate is the problem.
Projecting month-end spend, and the skew it adds
A projection turns the variance into a euro figure for the month:
projected month end = actual spend / days elapsed * 30.4
In the October example, €1,980 / 12 × 30.4 = €5,016. That is 10% above the €4,560 budget, the same 10% as the variance. With 30.4 on both sides, the projection always equals budget × (1 + variance). It adds no new signal. It restates the variance in euros, which is the unit your client reads.
It also carries the calendar skew, now in euros. At €165 a day, October really ends at €165 × 31 = €5,115, so the €5,016 projection is about 1.9% low. In February the same rate ends at €165 × 28 = €4,620, so a €5,016 projection would be about 8.6% high. For a calendar-true figure, use the real days:
calendar month end = actual spend / days elapsed * days in month
Both versions assume the rate so far continues. On the 3rd, that rate rests on three days, so treat an early projection as a rough guess.
Why does every window lean toward underspend until the day ends?
A window that ends today counts today as a full day of expected spend, though only part of today's spend has happened. The variance leans toward underspend until the day completes.
The shorter the window, the bigger the lean. Say 40% of today's spend is in. A client exactly on plan then shows -30.0% over 2 days, -4.3% over 14 days and -2.0% over 30 days. Late in a 31-day month, an even spender carries both effects and shows about -3.9% over 30 days.
Ending the window yesterday removes the lean. The price is one day of delay.
How Adswave applies the formula
The Budget Pacing view in Adswave runs this formula for every client. It shows expected against actual spend for rolling windows of 2, 14, 30, 90, 180 and 365 days ending today, plus a custom date range. Each window splits into Google Ads and Meta columns next to the total. The variance is green at 2% or less, amber up to 5% and red above 5%. Overspend and underspend get the same colours. The budget is the sum of the monthly budgets you enter per linked ad account in the Client dialog, under Admin, Clients. Each amount carries forward to later months until you change it. Adswave reads spend from Google Ads and Meta and never changes a budget or a campaign.
The view shows no month-to-date figure and no projected month-end spend. It counts today as a full day. It uses 30.4 in every month, so an evenly paced client shows about -1.9% in a 31-day month and about +8.6%, which is red, in February. Every window, the 365-day one included, is measured against the current month's budget. Pacing alerts apply the same formula to 1, 7, 14 or 30 complete days ending yesterday, so they skip the partial day. The free calculator adds a projection with a ±5% on-track band. Budget Pacing is on every plan, from €19.99 a month. The Budget Pacing feature page shows the view, and how budget pacing is calculated documents each step.
Questions people ask
What is the 30.4 rule in Google Ads?
Google Ads Help sets your monthly spending limit at 30.4 times your average daily budget for most campaigns. 30.4 is 365 divided by 12, the average month. Dividing a monthly budget by 30.4 therefore gives the daily budget whose monthly limit equals that budget. A €150 daily budget can spend up to €300 in a day and up to €4,560 in a month, for most campaigns.
How do I know if my campaign is overspending?
Work out the variance over a window of at least 14 days and compare it with your tolerance. A variance above it means you are spending ahead of the monthly plan. A single day at up to twice the daily budget sits inside Google's published limit, so one high day proves little. Check the calendar too: in February an evenly paced client shows about +8.6%. If a client is already ahead, the mid-month budget adjustment calculator gives the daily budget that lands the rest of the month.
How do you calculate budget pacing?
Divide the monthly budget by 30.4 to get the daily budget. Multiply it by the days in your window to get expected spend. Subtract expected from actual spend, divide by expected and multiply by 100. A €4,560 budget over 12 days gives €1,800 expected, so €1,980 of actual spend is +10.0%.
What is a good pacing percentage?
A pacing percentage is actual spend / expected spend × 100, so 100% is exactly on plan. The variance is that figure minus 100. Adswave's app shows 98% to 102% in green, the rest of 95% to 105% in amber and anything further out in red. The free calculator calls 95% to 105% on track. In February, an evenly paced client sits near 108.6%. A 2-day window swings far more than a 30-day one, so hold the longer windows to the tighter band.
Budget Pacing is on every Adswave plan. Start a 7-day free trial, no card required.
Google's spending limits are quoted from Google Ads Help, answer 6385083, checked 3 September 2026. Every other figure on this page is arithmetic you can recompute from the inputs shown.