Eggs: hatch in 8–20 hours in warm weather and must stay moist (UF/IFAS). Maggots: 4–13 days at warm temperatures, 14–30 days at 12–17 °C. Pupae: 2–6 days when hot, 17–27 days at about 14 °C. Egg to adult: about 10 days at 25 °C (Liu et al. 2026). Eggs per female: up to about 500. Empty or clean bins at least twice a week to break the cycle.
Life cycle: house fly in 4 stages (egg, maggot, pupa, adult)
The house fly goes through complete metamorphosis — egg, larva (the maggot), pupa and adult — and it does it fast. The house fly life cycle can be completed "in as little as seven to ten days" in warm conditions, or take up to two months when it's cool, and there can be 10–12 generations a year in temperate regions and more than 20 in the tropics (UF/IFAS).
| Stage | What it looks like | How long | Source |
|---|---|---|---|
| Egg | White, about 1.2 mm, laid in small piles of 75–150 on moist rotting material | 8–20 hours in warm weather | UF/IFAS; UNH |
| Maggot (3 instars) | Legless, cream-coloured, 3–9 mm early on and 7–12 mm when grown | 4–13 days at optimal temperatures; 14–30 days at 12–17 °C | UF/IFAS |
| Pupa | About 8 mm, inside a case that darkens from yellow to red, brown and black | 2–6 days at 32–37 °C; 17–27 days at about 14 °C | UF/IFAS |
| Adult | 6–7 mm, grey with four dark stripes on the thorax | Usually 15–25 days, up to two months | UF/IFAS |
Those are the fly life cycle stages you can actually see in a bin: tiny white eggs, wriggling white maggots, brown barrel-shaped pupae and finally the adult flies. The calculator turns the timings into dates for your temperature.
House fly life cycle days
Using the development model of Liu et al. (2026) — who reared house flies at constant temperatures from 16 to 34 °C — the whole cycle from egg to adult takes about 31 days at 16 °C (61 °F), 10 days at 25 °C (77 °F) and 6.2 days at 34 °C (93 °F). Development stops below about 11.4 °C. Those fly life cycle days match UF/IFAS's "seven to ten days" in warm weather.
How long does it take for fly eggs to hatch?
Hours, not days. "The legless maggot emerges from the egg in warm weather within eight to 20 hours" (UF/IFAS; UNH Extension gives the same 8–20 hours). How long does it take for fly eggs to hatch in a cool place? Longer: in our model, about 33 hours at 16 °C. There's one condition — "Eggs must remain moist or they will not hatch" (UF/IFAS).
| Temperature | Eggs hatch | Maggot stage | Pupa | Egg to adult |
|---|---|---|---|---|
| 16 °C (60.8 °F) | 33 hours | 20 days | 9.3 days | 31 days |
| 20 °C (68 °F) | 17 hours | 11 days | 5 days | 16 days |
| 25 °C (77 °F) | 11 hours | 6.7 days | 3.1 days | 10 days |
| 30 °C (86 °F) | 8 hours | 4.9 days | 2.3 days | 7.5 days |
| 34 °C (93.2 °F) | 6.6 hours | 4 days | 1.9 days | 6.2 days |
Egg-to-adult times are from the thermal-sum model of Liu et al. (2026); each stage is a fixed share of that total, taken from the middle of UF/IFAS's warm-weather ranges. So how long do fly eggs take to hatch on a warm day? About half a day — which is why food left out overnight in summer can have maggots by morning.
Maggots: how long until they become flies?
Maggots are house fly larvae. They feed in the material their eggs were laid in, moult twice (three instars) and grow to 7–12 mm with a "greasy, cream-colored appearance" (UF/IFAS). The maggot stage lasts 4–13 days at optimal temperatures but 14–30 days at 12–17 °C. High-moisture material helps them survive, and very little is needed: "sand or soil containing small amounts of degraded manure allows for successful belowground development" (UF/IFAS).
How long do maggots take to turn into flies?
About a week in warm weather. At 25 °C, maggots feed for about 6.7 days, then spend about 3.1 days as a pupa, so how long do maggots take to turn into flies? Roughly 9.8 days from hatching. How long are flies maggots in a cold bin? Weeks — up to a month at 12–17 °C (UF/IFAS).
When a maggot is fully grown it stops eating and leaves the food: it "can crawl up to 50 feet to a dry, cool place near breeding material" to pupate (UF/IFAS). That's why you sometimes find maggots or pupae under a bin rather than in it.
House fly pupae and adults
Fly pupae are about 8 mm long and form inside a hard case made from the maggot's last skin. The case changes colour "from yellow, red, brown, to black as the pupa ages" (UF/IFAS) — a quick way to tell an old pupa from a new one. Pupae complete their development "in two to six days at 32 to 37 °C, but require 17 to 27 days at about 14 °C". The emerging fly breaks open the case with a balloon-like sac on its head, the ptilinum, which it uses "like a pneumatic hammer" (UF/IFAS).
New adults are ready to breed quickly. They need food before they'll mate, females need protein to make eggs, and egg laying begins 4–20 days after mating (UF/IFAS). For how long the adults then live — usually 15–25 days — see the house fly lifespan calculator.
Where house flies lay eggs
Where do flies lay eggs? On moist, rotting organic material that their maggots can eat. In a Texas study, the best breeding sites were, in order, horse manure, human excrement, cow manure, fermenting vegetables and kitchen waste; fruit and vegetable piles, partly burnt garbage and incompletely composted manure are also "highly favored" (UF/IFAS). Fresh manure is especially attractive: UNH Extension notes eggs go into material less than a day old.
Fly eggs in trash cans and maggots in bins
Fly eggs in a trash can are usually laid on food waste stuck to the sides or bottom, or in an unsealed bag. Maggots in a bin mean the eggs had a week or so to develop — at 25 °C, the gap between egg and maggot is only about half a day, and maggots leave to pupate after about a week. Bins with tight-fitting lids that are cleaned regularly, and trash sealed in plastic bags, deny flies their breeding site (UF/IFAS).
- Kitchen bins and wheelie bins: wet food waste, especially meat and fish scraps.
- Compost: fresh scraps on top of a heap that isn't hot or turned.
- Pet waste: dog and cat faeces left in the garden, and litter trays.
- Food left out: any moist food a fly can reach.
How many eggs does a house fly lay?
Up to about 500 in a lifetime, in "several batches of 75 to 150 eggs over a three to four day period" (UF/IFAS). UNH Extension gives 75–150 per cluster and 350–900 over a female's life. Temperature changes the total: in the lab, females laid 184 eggs each at 20 °C, 729 at 25 °C, 709 at 30 °C and 506 at 35 °C (Fletcher et al. 1990). So how many eggs does a house fly lay? Hundreds — and often several females lay in the same spot, "leading to large masses of larvae and pupae" (UF/IFAS).
How fast do flies multiply?
In theory, terrifyingly fast. UF/IFAS cites a calculation that a single pair of flies starting in April could, if every egg survived, produce 191,010,000,000,000,000,000 descendants by August. That never happens, because most eggs and maggots die — but it explains how fast flies multiply when a bin is left for a fortnight in summer: at 25 °C, eggs laid on the first day are flying adults within about 10 days, and each female can lay hundreds of eggs.
How to break the house fly life cycle
Remove the breeding material faster than the flies can develop — no insecticide needed. These steps are from UF/IFAS and UNH Extension:
- Empty or clean bins at least twice a week in warm weather. Because the cycle can take as little as seven days, UF/IFAS advises removing wet waste at least twice a week to break it.
- Use tight-fitting lids on garbage cans and dumpsters, and clean them regularly (UF/IFAS).
- Bag and seal food waste before it goes in the bin (UF/IFAS).
- Keep bins away from doors — locate garbage receptacles "as far from building entrances as possible" (UF/IFAS).
- Turn the compost regularly so it composts properly; incompletely composted material is a favourite breeding site (UF/IFAS).
- Pick up pet waste and keep manure from piling up wet — spread or dry it where you can (UF/IFAS; UNH Extension).
- Screen windows and doors to keep adults out (UF/IFAS).
"Killing adult flies may reduce the infestation, but elimination of breeding areas is necessary for good management" (UF/IFAS). The calculator's timeline gives you the exact "clean the bin by" date for your temperature.
How this calculator works
- Egg to adult: the thermal-sum model of Liu et al. (2026): development needs about 3,350 degree-hours above a threshold of 11.43 °C, so time (hours) = 3,350.2 ÷ (temperature − 11.43). It was measured between 16 and 34 °C (706.5 hours at 16 °C and 163.6 at 34 °C), so the calculator accepts that range only.
- Stages: the total is shared out using the middle of UF/IFAS's warm-weather ranges — egg 14 hours (of 8–20), maggot 8.5 days (of 4–13) and pupa 4 days (of 2–6) — at every temperature, using the same stage-sharing engine as our fruit fly and mosquito life cycle tools. That's an assumption: real stages may not scale exactly together.
- Clean-the-bin date: the day the maggots finish feeding and leave to pupate.
- Egg hatch timer: the egg share of the same model, with non-toxic advice for where the eggs are.
The same model gives the egg-to-adult figure on the house fly lifespan page, so the two always agree. Dates are rounded to the nearest day; future dates are rejected.
Frequently asked questions
What is the life cycle of a house fly?
Egg, maggot (three instars), pupa and adult. It can take as little as 7–10 days in warm weather or up to two months when it's cool (UF/IFAS).
How long does it take for fly eggs to hatch?
8–20 hours in warm weather (UF/IFAS; UNH Extension) — longer when it's cool. The eggs must stay moist to hatch.
How long do maggots take to turn into flies?
About 9.8 days at 25 °C: roughly 6.7 days as a maggot and 3.1 days as a pupa.
How long are flies maggots?
4–13 days at warm, optimal temperatures and 14–30 days at 12–17 °C (UF/IFAS).
How many eggs does a house fly lay?
Up to about 500, in batches of 75–150 over three to four days (UF/IFAS); 350–900 in a lifetime according to UNH Extension.
Where do flies lay eggs?
On moist, rotting organic matter: manure, kitchen waste, fermenting vegetables, garbage and badly composted material (UF/IFAS).
Why are there maggots in my bin?
Flies laid eggs on food waste a week or so earlier. Use a tight lid, bag food waste and clean the bin at least twice a week in warm weather.
How fast do flies multiply?
Very fast: up to 500 eggs per female and a new generation in as little as a week, with 10–12 generations a year in temperate regions (UF/IFAS).
Do fly eggs hatch in the cold?
Slowly. Development stops below about 11.4 °C (Liu et al. 2026), and eggs that dry out won't hatch at all (UF/IFAS).
What do fly pupae look like?
Small brown barrel-shaped cases about 8 mm long that darken from yellow to red, brown and black as they age (UF/IFAS).
These calculators give estimates for education. Real flies vary with strain, food, humidity and temperature. For a stubborn infestation, your local cooperative extension service can help identify the insect. We give no pesticide advice.