How It Works
This calculator works in 4 steps.
Enter current fully diluted shares
Type all issued shares, granted options, and reserved shares.
Enter the existing unallocated pool
Type the unissued pool shares, or 0.
Enter the target pool percent
Type the target share of the post-expansion total, and choose pre-money or post-money placement for a financing round.
Read the results
The calculator returns new pool shares, the new total, and each holder's diluted percentage.
The Formula
The formula sizes the pool so it equals the target fraction of the post-expansion total.
S_total' = S_other / (1 − p)Pool = p × S_total'ΔS = Pool − Pool_existing- p:
- Target pool fraction
- S_other:
- All shares outside the unallocated pool
- Pool_existing:
- Current unallocated pool
With 10,000,000 shares and no pool, a 15% pool needs 10,000,000 / 0.85 − 10,000,000 = 1,764,706 new shares. Existing holders keep 85% of their prior percentages.
Conversion Table
The conversion table gives new pool shares and the remaining share for existing holders for 5 pool targets (starting from no pool).
| Shares before | Target pool | Pool shares | Total after | Existing holders keep |
|---|---|---|---|---|
| 10,000,000 | 10% | 1,111,111 | 11,111,111 | 90% |
| 10,000,000 | 15% | 1,764,706 | 11,764,706 | 85% |
| 10,000,000 | 20% | 2,500,000 | 12,500,000 | 80% |
| 8,000,000 | 10% | 888,889 | 8,888,889 | 90% |
| 20,000,000 | 12% | 2,727,273 | 22,727,273 | 88% |
What Is the Option Pool Dilution Calculator?
The Option Pool Dilution Calculator is a tool that returns the shares needed to create or expand an employee option pool to a target percentage, and the resulting dilution of existing holders. A pool at a target share of the post-expansion total needs more shares than the target percent of the current total.
To make a pool equal to 10% of the new total, the pool count is current shares × 10 / 90, not current shares × 10%. When an expansion happens as part of a financing round, the term sheet states whether the pool is counted in the pre-money shares (the "option pool shuffle"), which places the dilution on existing holders only. The calculator supports both placements.
Common Uses
There are 5 main uses of this calculator.
Hiring plans
Founders size the pool for the next 12 to 24 months of hires.
Financing negotiations
Founders model pre-money versus post-money pool placement.
Refresh grants
Companies size pool top-ups for existing staff.
Board approvals
Boards review dilution before approving a pool increase.
Employee communication
HR teams explain pool size and dilution.