What this calculator does
Most dogs cycle roughly every six months, though the interval varies from about four months in some small breeds to twelve in large ones. The cycle has four stages, and the two that matter for planning are proestrus, roughly days one to nine, and estrus, roughly days ten to twenty-one, which is when the bitch is fertile and receptive.
These are averages and individual dogs vary considerably. The fertile window in particular shifts by several days between dogs and between cycles in the same dog. Where timing actually matters, progesterone testing is the reliable method and this calculator is only a planning aid.
The formula
The next heat is the last start date plus the cycle interval entered. The stages are laid out from the last start date using the standard day ranges: proestrus days one to nine, estrus days ten to twenty-one, and anestrus covering the remainder until the next cycle. The stage boundaries are averages rather than fixed points.
| Term | Meaning |
|---|---|
| Proestrus | The first stage, roughly nine days, with swelling and bleeding but no receptivity. |
| Estrus | The fertile stage, roughly days ten to twenty-one, when mating can result in pregnancy. |
| Anestrus | The long quiet period between cycles. |
| Cycle interval | Time between the start of one heat and the start of the next, commonly around 183 days. |
The inputs explained
| Field | What to enter |
|---|---|
| First day of last heat | The start date of the last heat, taken as the first day bleeding was noticed. |
| Average cycle interval (days) | The cycle interval in days. Use the dog own history if you have it, since individual intervals are far more reliable than the average. |
When to use it
Planning around a season
Knowing roughly when the next heat is due helps with planning shows, travel, boarding and anything else that a season would interfere with.
Timing a planned mating
The estrus window gives a planning range, but progesterone testing is what actually pins ovulation. Use this to know which week to start testing, not to replace it.
Tracking an individual pattern
Recording start dates over several cycles gives a personal interval far more useful than the population average, and the calculator takes whatever figure you enter.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
When is the next heat due for each cycle interval?
The same last-heat date with three different cycle intervals.
| Cycle interval | Next heat expected around | Estrus, fertile window (days 10-21) | Anestrus (rest of cycle) |
|---|---|---|---|
| 150 days | 31 May 2026 | 10 Jan 2026 to 22 Jan 2026 | 22 Jan 2026 to 31 May 2026 |
| 183 days | 3 Jul 2026 | 10 Jan 2026 to 22 Jan 2026 | 22 Jan 2026 to 3 Jul 2026 |
| 210 days | 30 Jul 2026 | 10 Jan 2026 to 22 Jan 2026 | 22 Jan 2026 to 30 Jul 2026 |
Questions
How often do dogs come into heat?
Roughly every six months on average, but the range is wide. Small breeds often cycle every four to six months and large breeds every eight to twelve. Some breeds, notably Basenjis, cycle only once a year. An individual dog own pattern is usually consistent once established.
When is a dog most fertile?
During estrus, roughly days ten to twenty-one of the cycle, with peak fertility typically a couple of days after ovulation. The precise timing varies enough between dogs that progesterone testing is standard practice for planned breeding rather than counting days.
How long does a heat last?
The visible season is about two to three weeks, covering proestrus and estrus. Bleeding usually lasts around nine days and often eases as the fertile period begins, which catches people out: reduced bleeding does not mean the risk has passed.
When do dogs have their first heat?
Commonly between six and fifteen months, earlier in small breeds and later in large ones. The first few cycles are often irregular, so an interval calculated from them is less reliable than one taken from established cycles.
For gestation and due dates once a mating has happened, see the pet gestation calculator. For the cost of raising the resulting animals, see the pet cost calculator.