Predict her next heat, the fertile window, and the next 3 cycles
| Stage | Typical Length | What You See | Fertile? |
|---|---|---|---|
| Proestrus | ~9 days (3-17) | Swollen vulva, bloody discharge; males interested, she refuses them | No |
| Estrus | ~9 days (3-21) | Discharge lightens to pink/straw; she's receptive, "flagging" | Yes — days 9-14 is the usual window |
| Diestrus | ~60 days | Receptivity ends, discharge stops; false pregnancies show up here | No |
| Anestrus | ~4-5 months | Quiet recovery; the uterine lining resets | No |
Total heat as an owner experiences it (proestrus + estrus) runs 2-4 weeks. Add the ~60 days of diestrus and ~4-5 months of anestrus and the full cycle lands at 6-12 months — the interval the calculator uses.
| Breed Size | Examples | Typical Interval | First Heat Age |
|---|---|---|---|
| Small (<20 lb) | Chihuahua, Toy Poodle, Yorkie | 5-8 months | 6-10 months |
| Medium (21-50 lb) | Border Collie, Cocker, Beagle | 6-8 months | 8-12 months |
| Large (51-100 lb) | Lab, German Shepherd, Golden | 6-12 months | 10-18 months |
| Giant (>100 lb) | Great Dane, Newfoundland, St. Bernard | ~12 months, sometimes longer | 18-24 months |
Two famous exceptions cycle once a year: the Basenji (autumn season) and the Tibetan Mastiff. Individual dogs tend to be consistent once matured, which is why averaging her last two or three intervals beats any breed chart — including this one.
Intact female dogs are seasonal polyestrous breeders with no fixed calendar: they cycle on their own interval, roughly every 6 to 12 months, year-round. Prediction is therefore simple averaging — her next heat lands one typical interval after the last one started.
Next heat date = first day of last heat + average interval (in months). The fertile window = next heat start + 9 to + 14 days, because ovulation and receptivity (estrus) usually begin around day 9 of the cycle. The calculator projects three cycles ahead and flags two things worth a vet call: no heat for more than 12 months, or gaps consistently under 4 months (split heat patterns and some ovarian issues look like that).
Enter the first day you saw swelling or discharge — not the peak of the heat — and her interval if you know it from experience or records. New to this? Pick the size-based default, then update after her next cycle. The days-from-now counter tells you when to start watching for signs; most owners notice discharge 2-3 days after the true start.
A medium-sized Beagle whose heat started March 10, 2026 on a 6-month interval: predicted next heat September 10, 2026 — 12 days out from August 29. Her fertile window runs September 19-24, 2026 (days 9-14 of that cycle). Project further and cycle two lands March 10, 2027, cycle three September 10, 2027. If she was in heat 21 days or less ago, the calculator reports which stage she's likely in right now.
Breeders take this further: progesterone blood testing at the vet pinpoints ovulation to within a day or two, which matters because sperm survives in the tract for days and some girls ovulate early or late. The calendar narrows the search; chemistry closes it.
Most dogs cycle every 6 to 12 months, twice a year being the classic average for small and medium breeds. Small dogs often cycle every 5 to 8 months, large breeds 6 to 12, and giant breeds sometimes only once a year. A heat that started March 10, 2026 on a 6-month interval puts the next around September 10, 2026, then March 10, 2027.
About 2 to 4 weeks, with 18 days as the typical total: roughly 9 days of proestrus (swelling, bloody discharge, not receptive) followed by about 9 days of estrus (discharge lightens, she's receptive and fertile). Some dogs finish in 10 days, others stretch past 3 weeks, and first cycles are often irregular.
The fertile window is roughly days 9 to 14 counting from the first day of discharge and swelling, which is when most dogs ovulate and can conceive. On a heat that starts September 10, 2026, that's about September 19 to 24. Sperm survives days in the tract, so breeders treat the whole window as risky for an accidental breeding, and serious ones confirm ovulation with progesterone blood tests instead of the calendar.
Useful but not exact. Individual dogs are fairly consistent cycle to cycle, so averaging the last two or three intervals beats assuming 6 months, but a predicted date can drift 2 to 4 weeks either way. Re-enter each new cycle's first day to keep the projection honest, and treat any gap longer than 12 months (or a cycle much shorter than 4 months) as a reason to call the vet.
Usually 6 to 10 months in small breeds, and it can run 18 to 24 months in giant breeds. The first one or two cycles are often silent or irregular, which is normal. Keep an intact female away from intact males from her first heat onward, since fertility arrives immediately even in that first cycle.
Signs do the telling: proestrus brings a swollen vulva and bloody discharge with males attracted but her flagging them off; estrus lightens the discharge to straw-pink and she becomes receptive; diestrus ends receptivity as discharge stops over about 2 months. If discharge continues past 3 to 4 weeks, or smells foul, or appears with lethargy and drinking lots of water between heats, see a vet promptly, pyometra (uterine infection) is an emergency.
This tool estimates dates from averages and isn't veterinary advice. Cycle irregularities, unusual discharge, or a suspected pyometra need a vet, not a calculator.