Work backward from your enrollment goal. Enter where your study stands today to see the recruitment rate you need per site per month, how many pre-screened inquiries it will take, and what that could cost in ad spend. Add how long you have been recruiting and it also checks your current enrollment rate against the pace you need. Nothing you type is saved or sent anywhere.
Enrollment rate tells you how fast your study is actually filling. It is almost always expressed as participants enrolled per site per month, so studies of different sizes and lengths can be compared on the same scale.
To calculate it, take three numbers from your enrollment log: how many participants have consented and enrolled, how many sites have been actively recruiting, and how many months they have been open.
Example: a study has enrolled 18 participants across 3 sites in its first 6 months. That is 18 ÷ 3 ÷ 6 = 1.0 participant per site per month.
If sites opened at different times, count site-months instead of months: add up how many months each site has been open. Three sites open for 6, 4, and 2 months give 12 site-months, so 18 enrolled is 18 ÷ 12 = 1.5 per site per month.
Your current enrollment rate only matters next to the rate your timeline requires. Suppose the same study needs 60 participants in total and has 4 months left. It still needs 42 participants, so the required rate is 42 ÷ 3 sites ÷ 4 months = 3.5 per site per month. The study is running at 1.0, so it has to enroll 3.5 times faster to finish on time. That gap is what the calculator above shows when you fill in the pace fields.
Recalculate every two weeks. A falling enrollment rate is the earliest warning that a study is heading off schedule, usually weeks before anyone notices in the totals.
Three formulas sit behind the calculator. Together they turn an enrollment goal into a weekly target and a budget, and you can run them by hand for any study.
The pace your study needs from today, in participants per site per month. Months left is weeks left divided by 4.345.
Not everyone who inquires is eligible, consents, and enrolls. At a 10% rate, each participant takes about 10 pre-screened inquiries.
Optional. Use your own cost per inquiry from past campaigns, or a range, to see the budget the goal implies. Real benchmarks are in the clinical trial advertising cost guide.
A single-site study needs 40 more participants in 16 weeks. That is about 3.7 months, so the site needs roughly 10.9 participants a month. If 10% of inquiries enroll, the study needs 400 pre-screened inquiries, or about 25 a week. At $40 per inquiry, that is $16,000 in ad spend. At 15%, it drops to 267 inquiries and about $10,680. Small changes in the enrollment rate move the budget more than anything else, which is why fast follow-up on every inquiry matters.
The same 40 participants split across 2 sites in 16 weeks works out to about 5.4 participants per site per month. Adding sites lowers the rate each one needs, but it does not lower the total number of inquiries: 400 at a 10% enrollment rate, whether one site handles them or two. Recruitment advertising has to reach enough people around every site, not just the busiest one.
These terms get used loosely, and different sponsors define them slightly differently. Here is how they are used on this page and in the calculator.
| Term | What it measures | Formula |
|---|---|---|
| Recruitment rate | How fast people are brought into screening, or, when used loosely, the same as enrollment rate. Usually per site per month. | recruited ÷ sites ÷ months |
| Enrollment rate | How fast people consent and are enrolled. The number sponsors and PIs track most closely. | enrolled ÷ sites ÷ months |
| Required rate | The pace needed from today to hit the enrollment goal by the deadline. | still needed ÷ sites ÷ months left |
| Screen failure rate | The share of screened candidates who do not enroll. | (screened − enrolled) ÷ screened |
| Inquiry-to-enrollment rate | The share of pre-screened inquiries that end up enrolled. Drives how many inquiries your goal needs. | enrolled ÷ inquiries |
| Cost per inquiry | Ad spend divided by the pre-screened inquiries it produced. The planning number for budget. | ad spend ÷ inquiries |
A site screens 120 people and enrolls 30. Ninety failed screening, so the screen failure rate is 90 ÷ 120 = 75%, and the screening-to-enrollment ratio is 4 to 1. If the study still needs 20 participants, plan on screening about 80 more people, and on generating enough inquiries to fill those screening slots.
About 80% of clinical trials miss their original enrollment timeline, and Tufts CSDD found that reaching the goal typically takes about twice the planned time. Run the calculator with a shorter timeline than you have, and check it again every two weeks against real inquiry counts. The full numbers are on the clinical trial recruitment statistics page, clinical trial advertising explains how to reach the inquiries your goal needs, and the free recruitment plan template turns these figures into a week-by-week plan. If you are already behind, see clinical trial rescue.
Enrollment rate is the number of participants enrolled, divided by the number of active sites, divided by the number of months the study has been recruiting. For example, 18 participants enrolled across 3 sites over 6 months is 18 / 3 / 6 = 1.0 participant per site per month. To find the rate you need going forward, use participants still needed and months left instead.
Required recruitment rate = participants still needed / number of sites / months left to enroll. Months left is weeks left divided by 4.345. For 40 participants across 2 sites in 16 weeks (about 3.7 months), the required rate is roughly 5.4 participants per site per month.
The terms are often used interchangeably, and both are usually expressed per site per month. When teams separate them, recruitment rate counts people brought into screening (inquiries or screened candidates), and enrollment rate counts people who consented and were enrolled. The gap between the two is your screen failure and drop-off rate.
There is no universal benchmark, because the right rate depends on the protocol, the population, and the timeline. The useful comparison is your current rate against the rate your timeline requires. For context, Tufts CSDD found that 11% of sites in a typical trial enroll no one and 37% under-enroll against their target, so falling short of plan is common.
Divide 1 by your inquiry-to-enrollment rate. If 10% of pre-screened inquiries go on to enroll, you need about 10 inquiries per participant, so 40 participants take roughly 400 inquiries. Strict eligibility criteria, frequent visits, and slow follow-up all push the rate down. Use your own study history when you have it.
Screen failure rate = participants screened but not enrolled / total participants screened. If 120 people are screened and 30 enroll, 90 failed screening, so the screen failure rate is 75% and the screening-to-enrollment ratio is 4 to 1.
Use your own numbers from this study or a similar one at your site whenever possible. If you have none, the calculator starts at 10% as a planning placeholder, not a benchmark. Run it at a few rates, such as 5%, 10%, and 15%, to see the range of inquiries and spend you may need. For context, a 2017 JMIR review of Facebook recruitment in 35 health studies found that a median of 61% of people who responded to ads were eligible.
Multiply the number of inquiries you need by your expected cost per inquiry. Cost per inquiry depends on the audience, location, eligibility criteria, and how long the screener is. Clicks are far cheaper than inquiries: across nine Meta campaigns for university research studies, TrialFill paid an average of $0.42 per click, but only a share of clickers complete a screener.
No. The calculator runs entirely in your browser. Nothing you type is saved, sent, or stored.
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