Colony Growth & Cage Planner
Project how many animals and cages your colony will need over the coming months from your breeding pairs and litter rate.
Try Moustra freeHow the colony projection works
The calculator uses a straightforward deterministic model to estimate colony size and cage demand month by month. The core formula for animals weaned per month is:
weaned/month = breeding pairs × litter size × pre-weaning survival × (litters per year ÷ 12)
Each month, weaned animals are added to the standing (non-breeder) population. Monthly turnover is then applied as a fraction removed from that standing population — representing animals used in experiments, culled, or transferred out. If turnover is 0%, animals accumulate indefinitely; if it matches your weaning rate, the colony reaches a steady state.
Cage demand is calculated as the number of breeding pairs (one cage each) plus the ceiling of standing population divided by your animals-per-cage setting. This assumes breeders and non-breeders are housed separately, as is standard practice.
How to use this calculator
- Enter your breeding setup — number of breeding pairs, typical litter size for your strain, and how many litters you expect per year (most mouse strains: 5–8).
- Set pre-weaning survival — the percentage of pups you expect to survive to weaning. 90% is a good default for healthy inbred strains; lower this for strains with known perinatal issues.
- Set monthly turnover — the fraction of your non-breeder population removed each month through experimental use, culling, or transfers. Match this to your actual usage rate to get a realistic steady-state estimate.
- Set cage density — the number of non-breeder animals per housing cage allowed by your IACUC protocol or institutional guidelines (commonly 3–5 for mice).
- Read the monthly table — the table shows weaned animals, standing population, total animals (including breeders), and total cages needed at each month. Use the 6- and 12-month snapshots for quick capacity planning.
- Export or share — use the Export CSV button to download the full projection for your records or to paste into a budget spreadsheet, or copy the Share URL to send the same inputs to a colleague.
Frequently Asked Questions
How many cages will my colony need?
Cage demand depends on your breeding pairs plus the standing non-breeder population divided by cage density. This calculator estimates breeding cages (one per pair) plus housing cages (standing population ÷ animals per cage, rounded up). At typical mouse densities of 3–5 per cage and moderate turnover, a colony of 10 pairs will generally need 15–30 total cages by month 6.
How fast does a mouse colony grow?
A single breeding pair of mice can produce 6–8 pups per litter and have 5–8 litters per year, so the standing population can double every 1–2 months without active culling or use. The growth rate depends strongly on your monthly turnover (how many animals you use or transfer out). Setting turnover to match your expected use rate keeps the projection realistic.
What is a realistic weaning/survival rate?
Pre-weaning survival in healthy inbred strains is typically 80–95%. Outbred and hybrid strains often see higher survival. Losses are higher in first-time dams, during stress events, or in immunodeficient strains. The default of 90% is a reasonable starting point for a well-managed colony; adjust down if your strain has known pre-weaning issues.
How do I plan cage space for a growing colony?
Start by setting your projection horizon to 12 months and your monthly turnover to the fraction of animals you expect to use or transfer out each month. If the 12-month cage count exceeds your facility's capacity, you can increase turnover, reduce breeding pairs, or plan an expansion. Use the 6-month snapshot to identify when you'll first hit your cage limit.
What are the assumptions and limits of this model?
This is a deterministic model: it uses fixed average values for litter size and litters per year rather than simulating stochastic variation. It assumes breeding pairs produce at a constant rate from month 1. It does not model age-dependent fertility, genetic drift, or disease events. Use it for planning estimates and capacity budgeting, not precise predictions. For per-cage, per-litter tracking use Moustra.
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