Check a Lead File's Fill Rate Before You Pick the Merge Fields
Measure the columns first, then write the copy. Upload the lead file to the CSV fill rate checker and press Check fill rates. Every column comes back with its filled, blank and placeholder counts, its distinct and most common values, and a verdict: safe as a merge field, use with a fallback, too empty to personalise with, or the same value on every row. Only then decide which {{fields}} the sequence can lean on.
Sources, read 2 November 2026: Mailchimp's set default merge values article and Klaviyo's message personalization reference, which shows the default filter ({{ first_name|default:'friend' }}).
Why "looks full" isn't full
| What's in the cell | COUNTA says | A merge field prints |
|---|---|---|
| Blank | Empty | Nothing ("Hi ,") |
-, n/a, null, unknown |
Filled | The placeholder |
#N/A, #REF! |
Filled | The error text |
{{company}} left from another tool |
Filled | The literal tag |
| Same value on every row | Filled | A default the export filled in |
The checker counts the second to fourth rows as empty and flags the fifth, which is usually a broken export rather than real data.
Step by step
- Export the file you're about to send to, with every column.
- Upload it to the CSV fill rate checker and read the verdicts.
- Choose merge fields by verdict. Safe (98%+): use freely. Fallback (50 to 98%): write the sentence so the fallback reads naturally ("Hi there" works; "your company in your city" doesn't). Too empty (under 50%): don't build the opener on it, or use it only in per-row lines from the personalization line builder.
- Set the fallbacks where the tool keeps them. Mailchimp holds a default value on the audience field (every field except Email Address and Address); Klaviyo puts it in the tag with
|default:; other tools differ, and the merge tag converter notes which ones hold a fallback inline. - Fill what you can. Download the rows with gaps; the first name cleaner and company name cleaner fix the common ones.
- Check the tags against the rows. The mail merge field checker lists the exact rows each tag in your template would render blank for.
- Verify the email column on the verify page. Full is not the same as deliverable.
Honest limits
- Filled isn't correct. A cell can hold a real-looking value that's wrong or years out of date.
- Placeholders are a list, not a judgement. "Company" in a company column counts as filled.
- Report only. The checker doesn't change the file; it reports and exports the rows with gaps.
The longer guide to what goes wrong when fields are blank is mail merge blank fields.
Where SimpleVerifier fits
Once the columns are sorted, upload the file on the verify page; the first 100 checks are free with a Google sign-in. Pro is $39 a month with the first 100,000 checks a month at full speed, then slower, never cut off. See pricing.
Common questions
How do I count blank cells per column in Excel?
COUNTBLANK works per column for true blanks, but misses n/a, dashes and cells holding only spaces. The CSV fill rate checker counts blanks and placeholders for every column at once.
What fill rate is good enough for a merge field?
The checker uses 98%: on a 1,000-row list that's at most 20 rows on the fallback. Below that, write the sentence so the fallback reads naturally, or build per-row lines with rules.
Does it check whether the email addresses are valid?
No. It counts how full the email column is. Whether each address exists is a mailbox check, which the verify page does.
Is my file uploaded?
No. It is read in your browser and nothing is sent anywhere.
Check your list before you send
Real mailbox checks over SMTP, and nothing is sent to the address. Sign in with Google and your first 100 email checks are free, no card. Plans for bigger lists are on the pricing page.