Rat Breeding Calculator

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Rat Breeding & Litter Projection Calculator

Estimate gestation, weaning dates, and population growth

Breeding Estimates

Estimated Delivery Date:

Estimated Weaning Date:

Total Immediate Pups:

Projected Colony Size:

*Estimates based on an average 22-day gestation and 25-day weaning period. Colony size assumes 50/50 sex ratio and no mortality.

Understanding Rat Breeding Cycles

Breeding domestic rats (Rattus norvegicus) requires careful planning and timing. On average, a female rat (doe) has a gestation period of 21 to 23 days. This calculator helps breeders manage their space and resources by predicting when new litters will arrive and when they will be ready for separation from the mother.

Key Breeding Milestones

  • Conception: Occurs during the doe's estrus cycle, which happens every 4-5 days.
  • Gestation: Usually lasts 22 days. Signs of pregnancy often become visible around day 14.
  • Weaning: Pups should ideally stay with their mother for 21 to 28 days. Separation by sex is critical at 5 weeks to prevent accidental breeding.
  • Litter Size: While the average is 10-12, litters can range from as few as 1 to over 20.

How to Use This Calculator

To use the Rat Breeding Calculator, input the number of females you are currently breeding and your expected average litter size based on your line's history. Selecting the date of mating will automatically generate the estimated delivery date and the date the pups should be fully weaned and ready for new homes.

Colony Growth Projection

Rats are prolific breeders. If you select multiple breeding cycles, the calculator estimates potential population growth. For example, if you start with 2 does and each has 10 pups, and you continue for 3 cycles (assuming you keep the offspring for breeding), the numbers can escalate rapidly. This tool emphasizes why population control and responsible rehoming are essential in the rat hobby.

function calculateBreeding() { var numDoes = parseFloat(document.getElementById('numDoes').value); var avgLitter = parseFloat(document.getElementById('avgLitter').value); var dateInput = document.getElementById('conceptionDate').value; var cycles = parseInt(document.getElementById('numCycles').value); if (isNaN(numDoes) || isNaN(avgLitter) || !dateInput || isNaN(cycles)) { alert("Please enter all breeding details to calculate."); return; } // Date Logic var conception = new Date(dateInput); // Delivery (approx 22 days) var delivery = new Date(conception); delivery.setDate(delivery.getDate() + 22); // Weaning (approx 25 days after birth) var weaning = new Date(delivery); weaning.setDate(weaning.getDate() + 25); // Calculations var immediatePups = numDoes * avgLitter; // Compound growth logic (simplified for hobbyist tracking) // Formula: Current * (1 + (LitterSize/2))^Cycles // Assumes half are female and joined the breeding program each cycle var projectedGrowth = numDoes; for (var i = 0; i < cycles; i++) { projectedGrowth = projectedGrowth + (projectedGrowth * avgLitter); } // Display document.getElementById('deliveryDateVal').innerText = delivery.toDateString(); document.getElementById('weaningDateVal').innerText = weaning.toDateString(); document.getElementById('totalPupsVal').innerText = immediatePups + " Pups"; document.getElementById('projectedGrowthVal').innerText = Math.round(projectedGrowth) + " Rats Total"; document.getElementById('breedingResult').style.display = 'block'; } // Set default date to today window.onload = function() { var today = new Date().toISOString().split('T')[0]; document.getElementById('conceptionDate').value = today; };

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