"How many subs do we need?" is a question I get constantly, and the answer is always "it depends." But it depends on factors you can measure, which means you can calculate a specific number rather than guessing. Most districts either have too few subs (resulting in low fill rates) or too many on their roster (creating the illusion of capacity while most subs are inactive).
The optimal substitute pool size is determined by four factors: your average daily teacher absences, your target fill rate, your pool activation rate (the percentage of rostered subs who actually accept assignments regularly), and your peak absence multiplier. The formula is: Target Pool Size = (Peak Daily Absences x Target Fill Rate) / Pool Activation Rate. Plug in your own district's numbers to get a realistic target. Most districts underestimate the activation rate gap: having a large roster on paper means nothing if only a fraction of those subs accept assignments regularly.
The four inputs you need
1. Average daily absences
Pull absence data from the previous year. Calculate the average number of teacher absences per day. Then calculate the peak: the highest single day and the average of the top 10% of absence days. Your pool needs to handle the peak, not the average.
Most districts find their peak absence days are 40-60% higher than their average. If your average is 30 absences per day, your peak days likely hit 42-48.
2. Target fill rate
What percentage of absences do you want covered by a substitute? Most districts target 85-95%. A 100% target is unrealistic due to same-day absences, hard-to-fill positions, and days when even large pools are fully utilized.
Set your target based on what is acceptable for your schools. Below 80% creates chronic coverage problems. Above 95% requires a very large, active pool.
3. Pool activation rate
This is the most important and most misunderstood number. Of all the subs on your roster, what percentage accept at least one assignment per month? In most districts, this number is 30-50%. A roster of 200 subs with a 35% activation rate is functionally a pool of 70.
Measure this by reviewing assignment data. Count the number of unique subs who accepted at least one assignment in the last 30 days. Divide by total rostered subs. That is your activation rate.
4. Peak absence multiplier
Your pool needs to handle not just average days but the worst days. Apply a multiplier based on your peak-to-average ratio. If your peak absence days are 50% higher than average, your multiplier is 1.5.
Running the calculation
The formula
Target Pool Size = (Peak Daily Absences x Target Fill Rate) / Pool Activation Rate
Example: Medium-sized district
- Average daily absences: 35
- Peak daily absences (top 10% average): 52
- Target fill rate: 90%
- Pool activation rate: 40%
Target Pool Size = (52 x 0.90) / 0.40 = 117 subs
This district needs 117 subs on the roster to cover 90% of absences on peak days, given that only 40% of rostered subs are regularly active.
Example: Small district
- Average daily absences: 8
- Peak daily absences: 14
- Target fill rate: 85%
- Pool activation rate: 50%
Target Pool Size = (14 x 0.85) / 0.50 = 24 subs
Smaller districts need proportionally larger activation rates because the margin for error is smaller. One inactive sub in a 24-person pool has more impact than one in a 117-person pool.
What to do with the number
If your pool is too small
Recruit aggressively. Focus on the channels that produce the most active subs, not just the most applications. A sub who signs up and never works does not help your numbers.
If your pool is the right size but your activation rate is low
The problem is not pool size. It is engagement. Survey inactive subs. Why did they stop accepting assignments? Bad experiences? Low pay? Scheduling conflicts? Address the root cause rather than recruiting replacements who will become inactive for the same reasons.
If your pool is too large (on paper) but fill rates are low
Your activation rate is the problem. A bloated roster of inactive subs creates a false sense of security. Focus on activating existing subs rather than adding more names to a list nobody checks.
Recalculate every semester
Absence patterns change. Sub pool composition changes. Activation rates fluctuate. Recalculate your target pool size at the start of each semester and adjust your recruitment and retention strategies accordingly.
What to measure
- Rostered pool size vs. active pool size (close the gap between paper and reality)
- Pool activation rate (track monthly, target 50%+)
- Peak day coverage (what is your fill rate on the 10 worst absence days?)
- Recruitment yield (of new subs onboarded, what percentage become active within 30 days?)
- Pool composition (full-time subs, part-time, retired teachers, college students, etc.)
Common mistakes
- Confusing rostered pool size with active pool size. The number that matters is how many subs actually work, not how many are on the list.
- Sizing for average days instead of peak days. Average days are manageable. Peak days break your system. Size for the peak.
- Not tracking activation rate. If you do not know what percentage of your pool is active, you cannot assess whether your pool is adequate.
- Recruiting without retaining. Adding 50 new subs while losing 40 existing ones nets you 10. Fix retention before scaling recruitment.
If you only do one thing this week: Calculate your pool activation rate. Pull the list of every rostered sub. Count how many accepted at least one assignment in the last 30 days. Divide. If the number is below 40%, your pool is smaller than you think, and your fill rate reflects the active pool, not the roster.
