Project month-by-month mouse colony growth from a small founder population. Generations to target, peak cage count, and a 12-18 month colony curve — with optional facility cap.
Try it out
Load example colony expansion forecaster data to see the full workflow
Number of founder pairs at month 0.
Use the average pup count *at weaning*, not at birth.
Default 0.5 (mouse sex ratio is approximately 1:1).
Fraction of weanlings retained (default 0.9 for healthy SPF colonies).
Mating to next mating. Default 11 weeks for B6-class strains.
How many months to project. Default 18 ≈ 7 generations.
The forecaster reports the first generation that hits this number.
Growth is clipped at this number; capped generations are flagged in the table.
| Gen | Months | Pairs | Weanlings | New pairs | Live colony | Cages |
|---|---|---|---|---|---|---|
| G0 | 0 | 5 | 0 | 0 | 10 | 5 |
| G1 | 2.5 | 12 | 31 | 7 | 41 | 17 |
| G2 | 5.1 | 30 | 75 | 18 | 116 | 44 |
| G3 | 7.6 | 77 | 189 | 47 | 305 | 115 |
| G4 | 10.1 | 198 | 485 | 121 | 790 | 297 |
| G5 | 12.7 | 509 | 1247 | 311 | 2,037 | 764 |
| G6 | 15.2 | 1310 | 3206 | 801 | 5,243 | 1966 |
| G7 | 17.7 | 3373 | 8253 | 2063 | 13,496 | 5061 |
Amber rows: growth was clipped by the facility cap that generation. Green colony numbers: target population reached.
When to use
Do not use for
Published B6 litter sizes are 6-8 born per litter, but only 5-7 wean reliably. Plug your *weaned* litter size into this calculator, not the live-birth count. Otherwise the curve will run hot and your real cage need will be 20-30% lower than the projection.
When the calculator forms 7 new pairs in a generation, those 14 mice are part of the same 31 weanlings — they are not extra animals. The live colony grows by *weanlings retained*, not by weanlings + new pairs. This is the most common forecasting bug in handmade spreadsheets.
The first batch of pups appears at generation 1, which is one full generation cycle (default 11 weeks 2.5 months) after you set up the founder pairs. If your timeline is "200 mice by month 6," remember the *first* productive generation is at month 2.5.
A 200-cage cap means the simulation will clip growth — but in real life, that clipping is a person culling weanlings every Monday. Make sure your IACUC protocol covers the routine cull and that the SOP is current.
Discrete-time generation simulation. Each generation g: weanlings = active_pairs litter_size retention_rate; new_pairs = floor(weanlings female_ratio / 2); active_pairs_(g+1) = active_pairs_g + new_pairs; live_colony_(g+1) = live_colony_g + weanlings (clipped at facility capacity if set). Generation 0 is the founder month (no pups yet). Default cycle is 11 weeks per generation; horizon defaults to 18 months. Cage accounting: 1 breeding cage per active pair + ceil(held_weanlings / 4) holding cages.
Last validated 2026-04-07. Calculations are designed for planning and documentation support; verify procurement decisions against manufacturer specifications or institutional SOPs.
ConductScience Colony Expansion Forecaster (v1.4.0). ConductScience, Inc. 2026. Available at: https://conductscience.com/tools/colony-expansion-forecaster
National Research Council. Guide for the Care and Use of Laboratory Animals. 8th ed. Washington, DC: The National Academies Press; 2011.
Whittaker DE, Kasper C, Brust V, Würbel H, Voelkl B. Good practice in the management of laboratory rodent breeding colonies. Lab Anim. 2016;50(4):258-264.
Most labs scale up colonies "by feel." You set up 5 pairs in January, glance at the rack in March, and panic in June when you need 200 mice for an experiment but only have 60. Or worse: you panic because you have 800 mice and no room.
A discrete-time projection. Run the math forward from your real founder count, your real litter size, and your real retention rate. The output is a month-by-month curve and a generation table — exactly the surface you can present to a facility manager or PI to justify cage allocation.
Geometric growth dominates the first ~6 generations. After that, either the facility cap or breeder retirement flattens the curve.
Every facility has a hard cage limit. When the colony hits the cap, you cull weanlings — usually the surplus males and the late-litter pups. Most labs do this *unintentionally* (oops, no clean cages this week) instead of *intentionally* (here is the plan).
1. Run with no cap → see the unconstrained biological ceiling. This is what your colony would do in a perfect rack. 2. Run with your real cap → see the month the cull starts and the steady-state population the cap supports. 3. Compare the two curves to budget cull rates and to justify a facility expansion if needed.
A capped colony is also a *managed* colony — you are committing to cull every generation past the cap, which has IACUC, ethical, and per-diem implications. The 3Rs Justification Builder (coming in Phase 26) helps articulate the trade-off in writing.
Reads your active cages and runs the same forecast forward from today, generation by generation, with auto-updating charts as your real breeding lands.
Open app EquipmentPlan how many trios and how many cages each generation needs to deliver enough weanlings for the projected growth curve.
Browse EquipmentTranslate the projected colony into NIH-Guide-compliant cage counts and check facility utilization.
Browse QuoteGet a quote on equipment or talk to the ConductScience team.
Request quote