Why divide by 30.4? The Google Ads 30.4-day month

The 30.4-day month is the average month, 365 days divided by 12 and rounded to 30.4, used to turn a monthly budget into a daily one.

You divide a Google Ads budget by 30.4 because Google's monthly spending limit is 30.4 times your average daily budget. A daily budget of monthly / 30.4 makes that limit equal your monthly figure in every month. The cost is a calendar mismatch. A client spending evenly reads about -1.9% in a 31-day month and about +8.6% in a 28-day February.

What Google says about the monthly limit

Google publishes the monthly ceiling in its own help centre:

Your monthly spending limit (30.4 times your average daily budget for most campaigns) in any particular month.

Source: Google Ads Help, answer 6385083, checked 3 September 2026.

The formula

30.4                = 365 / 12 (30.42), rounded to one decimal
daily budget        = monthly budget / 30.4
Google's monthly    = daily budget * 30.4  (= the monthly budget)
expected spend      = monthly budget / 30.4 * days in the window
even-pace variance  = 30.4 / days in the month - 1

A worked example in euros

A client on €3,000 a month gets a daily budget of €98.68 (€3,000 / 30.4). Google's monthly limit is 30.4 times that, which is €3,000 again. Over a 14-day window, expected spend is €1,381.58.

Now let that client spend evenly across the calendar month. On a 14-day window inside the month, the variance depends only on the month's length:

In a 31-day month the expected line runs about 2.0% above an even calendar split, so steady spend reads as underspend. In a 28-day February it runs about 7.9% below, so the same steady spend reads as overspend. February 2027 has 28 days. February 2028 has 29.

Why not divide by the days in the month?

You can, but then Google's limit moves instead of your variance. Divide €3,000 by 28 in February and the daily budget is €107.14. Google's monthly limit becomes €3,257.14, which is 108.6% of the agreed figure. Divide by 31 and the limit falls to €2,941.94, or 98.1%. Dividing by 30.4 keeps the limit on the number you answer to the client for.

Where the 30.4 figure misleads

• February looks like overspend. An on-plan client turns red because the month is 2.4 days shorter than 30.4, not because anything overspent.
• Today counts as a whole day. In the pacing view, today is a full day of expected spend while its spend is still partial. Every window leans toward underspend until the day completes. The table assumes complete days.
• Windows that reach into last month blend two months. During February the 30-day window always reaches into January but is measured against February's budget. With equal budgets, its variance climbs through the month instead of starting at +8.6%.
• "For most campaigns." Google's sentence carries that qualifier, so check its page for your campaign type.

How Adswave computes it

Every expected-spend figure in Adswave divides the monthly budget by 30.4, whatever the calendar month. No setting switches to calendar days. Meta budgets get the same 30.4.

The Budget Pacing view shows, per client, expected against actual spend for rolling windows of 2, 14, 30, 90, 180 and 365 days ending today, plus a custom range. Expected is monthly budget / 30.4 x days in the window. The variance is green at 2% or less, amber up to 5% and red above 5%, over and under alike.

Pacing alerts use the same expected line over complete days ending yesterday, so today's partial spend never enters them. A 5% pacing rule set to up or either fires on an evenly paced client once its window sits inside a 28-day February. From the 30.4-day month alone, a 10% rule stays quiet. Budget Pacing and alerts are on every plan.

The Google Ads daily budget calculator sets a daily budget the same way. Expected spend to date covers the expected line, and how budget pacing is calculated shows the full arithmetic.