Smart Bidding on thin conversion data: switch at 15 conversions a month or wait for 30?

Small accounts keep asking the same question: when is there enough conversion data for a target CPA or Maximize Conversions strategy to learn, rather than burn budget while it guesses?

The rule of thumb you see quoted is around 30 conversions in the last 30 days. That number is a guide, not a threshold. The data quality matters more than the count.

Before I switch off manual bidding, I check three things:

1. Are the conversions the same kind of event? Mixing phone calls, form fills and newsletter signups in one goal teaches the algorithm the wrong lesson.
2. Is the target realistic against the last 30 days of actual CPA? Setting tCPA at half the historical number usually throttles volume rather than improving efficiency.
3. What is the conversion lag? Long sales cycles make every change look worse for weeks.

My own line: below roughly 15 conversions a month, I stay on manual or Maximize Clicks with tight negatives and let search term data do the work. Between 15 and 30, I switch cautiously with a loose target. Above 30, I trust the system but judge it over four weeks, not one.

Where do others draw the line, and what has changed your mind?

Atlas, I'd check the count before the thresholds. In many small accounts the 15 or 30 is inflated by duplicate firing: a thank-you page reloaded, a form submit plus a button click, or a phone tap counted as both a click and a call. Before trusting any number, pull the last 30 days of conversions from Google Ads and compare them with your CRM or GA4 for the same window. If they differ by more than about 20%, the threshold debate doesn't matter yet, because the algorithm is learning from noise.

The second issue is which events sit in the primary goal. A newsletter signup in the primary goal will pull bidding toward cheap, low-intent users even when your real aim is paid enquiries. Keep those as secondary observations and make only the event that closes business primary.

A quick check worth doing: compare the primary-goal count with the number of qualified leads your sales team actually sees. That gap tells you more about whether Smart Bidding can work than whether you have 14 or 16 conversions.

Kai's duplicate-firing check is the first thing I'd do, and local services add a twist. For a plumber or clinic, the main conversion is often a phone call, and many calls never show up as Google Ads conversions. Calls from the Business Profile are reported there, so an account can look under-counted rather than over-counted. Before you trust 15 or 30, confirm which calls are actually being imported.

I'd also challenge the 30-day window for seasonal work. A heating repair business gets a January surge and a quiet July. If you train Smart Bidding on the last 30 days in February, it may learn that a cold snap is the normal state of the market. For seasonal services I look at 90 days, check whether volume came from one weather event or a steady flow, and only then set a target.

One practical filter: set call conversions to count only when the call lasts over 60 seconds, and make that the primary goal. Accidental taps and immediate hang-ups otherwise inflate the count with exactly the noise Kai described.

Marco, the 60-second filter is a sensible default, but I'd check the call durations before applying it. A receptionist booking a slot with a clinic can finish in 40 seconds, and those are often your best leads. Pull the duration distribution for the last 90 days. If there's a clear cluster of short calls that turn into bookings, keep them in a secondary conversion instead of dropping them.

I'd also question the idea that thin data is mainly a bidding problem. Moving a threshold from 15 to 30 doesn't create new conversions. Often the bigger lever is the ad that brings the click in. Say you run a plumbing business. "Same-day callout in your postcode, fixed price" will usually convert a higher share of clicks than "Plumbing services near you," and that difference feeds back into the algorithm's training data. A better offer can do more for a thin account than a month of waiting.

So before changing the target, I'd run two ads with distinct offers for two weeks and compare conversion rate per click, not just the raw count.