Fungus gnats: adults live 7–10 days (University of Kentucky); egg to adult takes about 17 days at 24 °C (UC IPM) and about 18–25 days at 21 °C. The problem: lasts until the wet soil is dealt with — after repotting, the last adults should be gone in about 10 days. Drain flies live only a few days as adults (see the drain fly lifespan calculator).
How long do gnats live?
Not long. The gnats in most homes are fungus gnats, and University of Kentucky entomologists give their adult lifespan as just 7–10 days. So how long can a gnat live? About a week and a half at most as a flying adult — the fungus gnat lifespan is one of the shortest of any household insect. Drain flies, which people also call gnats, live only a few days; the drain fly lifespan calculator covers them.
That short adult life is why the gnat life span surprises people. Each adult lives about a week, but new ones keep hatching from eggs in wet soil, so a gnat problem that has lasted two months is many generations of gnats, not the same ones. A fungus gnat goes from egg to adult in about 18–25 days at a room temperature of 21 °C (70 °F), so the young are always a step behind the adults you see.
In short, how long gnats live as adults is measured in days, but how long a gnat problem lasts is measured in weeks — until the breeding site dries up or is cleaned. People ask it many ways — how long can gnats live, how long is the lifespan of a gnat, what is the gnat life expectancy — and the answer is the same: 7–10 days for the adult, longer for the problem.
How long does a fungus gnat live?
How long do fungus gnats live? Adults 7–10 days (University of Kentucky). Adult fungus gnats "feed very little, consuming only liquids, such as water or flower nectar" (Missouri Botanical Garden), so they spend that short life mating and laying eggs — 100–200 per female, on moist soil or near plant stems (University of Kentucky). How long does a fungus gnat live if you count the time underground as well? Roughly 25–35 days at 21 °C from egg to the end of adult life, because most of the life span of a fungus gnat is spent as a larva and pupa in the soil.
What kind of gnat do you have?
"Gnat" isn't a scientific name — it's an everyday word for many kinds of tiny fly. That's the whole story of fungus gnats vs gnats: fungus gnats are one kind of gnat, and indoors they're the kind you're most likely to have. The University of Kentucky lists fruit flies, fungus gnats, drain flies and phorid flies as the small flies found most often in homes, and they live very different lives:
| Insect | Size and look | Where you see it | How it flies |
|---|---|---|---|
| Fungus gnat (Sciaridae) | 1/16–1/8 in; dark, delicate, mosquito-like; antennae longer than the head (UC IPM) | Around houseplants and on moist potting soil (UC IPM) | Weaker fliers than look-alike shore flies (UC ANR) |
| Drain (moth) fly (Psychodidae) | 1/16–1/4 in; hairy, moth-like; wings held roof-like over the body at rest (Clemson HGIC) | Near sinks, showers and floor drains (Clemson HGIC) | Weak fliers that seem to hop (Terminix, a pest-control company) |
| Fruit fly (Drosophilidae) — fruit fly lifespan | About 1/8 in; tan front, black rear, red eyes (University of Kentucky) | Around ripe fruit, bins and recycling (University of Kentucky) | Drawn to the smell of fermenting food |
So gnats vs fungus gnats isn't a choice between two insects: if your "gnats" rise from a flowerpot, they are fungus gnats, and the lifespan above applies. If you're not sure which you have, the fruit fly vs gnat vs drain fly identifier asks five quick questions with picture cards; found worms rather than flies? Try the larvae identifier.
Gnat life span: adults vs the whole infestation
The adult is only the last stage of a fungus gnat's life. Before it, the gnat spends days as an egg, a larva and a pupa in damp soil, and warmth speeds all of it up: a 2019 study found that female Bradysia impatiens took 93 days to develop at 11 °C and just 21 days at 25 °C (Ishihara & Tagami 2019). The gnat life cycle calculator gives the date of every stage at your temperature.
| Temperature | Egg to adult | Egg to end of adult life |
|---|---|---|
| 11 °C (51.8 °F) | 93–113 days | 100–123 days |
| 16 °C (60.8 °F) (calculated) | 46–57 days | 53–67 days |
| 19 °C (66.2 °F) | 18–23 days | 25–33 days |
| 22 °C (71.6 °F) (calculated) | 17–25 days | 24–35 days |
| 24 °C (75.2 °F) | 17–27 days | 24–37 days |
| 25 °C (77 °F) | 16–27 days | 23–37 days |
So how long is a gnat's lifespan from egg to death? For a fungus gnat at room temperature, roughly 25–35 days. The infestation, though, lasts as long as the soil stays wet, because every female adds a new batch of eggs before she dies.
How long do gnats live in your house?
How long do gnats live in your house? Each adult lives about a week — so the answer to "how long do gnats live in house plants and drains" is really about the breeding site. What matters indoors is whether they can breed. Fungus gnats need "moist to very moist soil or substrate" (UF/IFAS), so overwatered pots, poorly drained containers and damp organic matter keep them going. Drain flies breed in the gelatinous film inside drains and sewers (Clemson HGIC).
- How long do gnats live in plants? As long as the potting soil stays wet and full of organic matter, the colony can carry on indefinitely, with a new generation every 2–4 weeks.
- Gnats in potting soil come from eggs laid on the surface; the larvae have shiny black heads and clear to white bodies and feed on fungi, organic matter and sometimes roots (UC IPM).
- Gnats in the bathroom or kitchen sink are usually drain flies, breeding in slow or seldom-used drains.
So how long do gnats live indoors? A week or so each, with the infestation lasting as long as the source does. Why do I have gnats in my house? Almost always because a pot is staying too wet or a drain needs cleaning. Where do gnats come from in the first place? Adults can get in through gaps — UC IPM advises screening and caulking leaky windows and doors — and fungus gnats also arrive in compost-rich potting mix that wasn't pasteurised (UC IPM).
How long do gnats live without food?
How long do gnats live without food? About as long as they would anyway. Adult fungus gnats "feed very little, consuming only liquids, such as water or flower nectar" (Missouri Botanical Garden), so taking away food doesn't shorten their 7–10 day life by much. We couldn't find a study that measured how long fungus gnat adults survive with no food or water at all, so the calculator doesn't guess; it uses the normal adult lifespan for the "no food or breeding site" situation.
What does make a difference is taking away the place they breed. With no wet soil or drain slime, no new gnats can develop, and the adults you can see should be gone within about a week (fungus gnats) or a few days (drain flies). The larvae are another matter: they live in their food, so as long as the soil stays damp they keep eating and growing.
How long does a gnat infestation last?
How long does a gnat infestation last — and how long does it take to get rid of gnats — once you fix the source? Count two things: any larvae and pupae already developing, and the adults' remaining life. At 21 °C (70 °F), the calculator's worst-case estimates are:
- Let the topsoil dry: up to about 34.6 days — larvae deeper in the pot may still finish developing (18–25 days) before the last adults live out their 7–10 days.
- Replaced or repotted the soil: about 10 days — the eggs, larvae and pupae go out with the old soil, so only adults remain.
- Cleaned the drain (drain flies): about 9 days — pupae can still emerge for a day or two, then adults live up to a week.
If gnats are still around well after those dates, there's another source: a different pot, a saucer of standing water, a second drain, or wet mulch and compost. The calculator's second mode gives the date and a timeline for your situation.
Will gnats go away on their own?
Will gnats go away on their own? Only if their breeding site does. Indoors, fungus gnats have continuous, overlapping generations all year (NC State Extension), so a pot that stays wet keeps producing them. Let the surface dry and the larvae die — "when the soil dries, the larvae will die" (University of Maryland Extension) — and then how long does it take for gnats to die? The adults you can see are gone within their 7–10 day lifespan, which is why the dates above add that lifespan to the time any larvae need to finish developing.
How to get rid of gnats for good
Extension services agree that controlling the breeding site matters more than killing adults. These non-toxic steps are from UC IPM, UF/IFAS, the University of Kentucky and Clemson HGIC:
- Let the surface of the soil dry between waterings, avoid overwatering and improve drainage, for example with perlite or sand (UC IPM).
- Use yellow sticky traps cut into small squares on sticks in the pots to catch adults and show where they're coming from (UC IPM).
- Check for larvae with potato: push raw potato chunks, cut side down, into the soil and look underneath after a few days (UC IPM).
- Repot in pasteurised potting mix and clear away rotting leaves and plant debris (UC IPM; UF/IFAS).
- Empty saucers and fix leaks so there's no standing water (UC IPM).
- Gnats around a drain? Those are drain flies: scrub the slimy film out with a stiff brush (Clemson HGIC) — see drain flies vs gnats for the overnight test that finds the drain.
UC IPM also lists biological options that target larvae, such as Bacillus thuringiensis israelensis (Bti) and beneficial nematodes. We don't recommend chemical sprays: they only kill the adults present and don't fix the breeding site.
How this gnat lifespan calculator works
Gnat lifespan mode shows the sourced adult lifespan for your gnat type (fungus gnats 7–10 days; drain fly figures are the same ones the drain fly lifespan calculator uses) and the egg-to-adult time at your temperature. Fungus gnat development uses measured values at 11, 17–21 (plotted at 19), 24 and 25 °C, with straight lines between them; no development time above 25 °C was found, so warmer rooms use the 25 °C figures and the result says so. Drain fly stage times were measured at about 21 °C and are shown with a note. "Not sure" uses where you see them: near plants means fungus gnats, near drains means drain flies, and near fruit points you to the fruit fly page.
Infestation mode adds a worst case to the date you fixed the source: drying the soil allows for the slowest larvae and pupae to finish developing, replacing the soil leaves only adults, and cleaning a drain allows for pupae to emerge (up to 2 days). Then it adds the longest sourced adult lifespan. Future dates are rejected. The temperature must be 11–30 °C (52–86 °F), the range of the fungus gnat studies.
Frequently asked questions
How long do gnats live indoors?
Adult fungus gnats live 7–10 days (University of Kentucky). Indoors, new ones keep emerging as long as the soil — or, for drain flies, the drain — stays wet.
How long can gnats live without food?
About their normal adult lifespan — roughly a week for fungus gnats. Adults take only liquids such as water or nectar (Missouri Botanical Garden); no study measured survival with no food at all.
What is the life expectancy of a gnat?
About a week as an adult. From egg to the end of adult life, a fungus gnat lives roughly 25–35 days at 21 °C.
Are fungus gnats the same as gnats?
Fungus gnats are one kind of gnat — the one in most homes. "Gnat" is an everyday word for many small flies; fungus gnats are the dark, mosquito-like ones that breed in houseplant soil.
Will gnats go away on their own?
Only if the breeding site dries up. Indoors fungus gnats breed all year in wet soil (NC State Extension); let the surface dry between waterings and the larvae die (University of Maryland Extension).
How long does it take to get rid of gnats?
About 10 days after repotting, or up to about 34.6 days after you start letting the topsoil dry, at 21 °C — the time for larvae to finish developing plus the adults' 7–10 days.
How long does a gnat infestation last?
Until the breeding site is fixed. After replacing the soil, expect about 10 days; after drying the topsoil, up to about 34.6 days at 21 °C.
How long do fungus gnats live?
The life span of fungus gnat adults is 7–10 days (University of Kentucky), after spending two to four weeks as eggs, larvae and pupae in the soil at room temperature.
How long can a gnat live?
An adult fungus gnat lives 7–10 days (University of Kentucky). Counting the egg, larva and pupa in the soil, about three to five weeks at room temperature.
How long does it take for gnats to die?
Once no new ones are hatching, the adults die within their 7–10 day lifespan. With no breeding site, expect the last adults within about a week and a half.
How long does a fungus gnat live?
As an adult, 7–10 days (University of Kentucky); from egg to the end of adult life, roughly 25–35 days at 21 °C.
How long do gnats live in plants?
Each adult lives about a week, but a colony in wet potting soil can continue indefinitely, with a new generation every few weeks.
Why do I have gnats in my house?
Usually because potting soil stays too wet or a drain has built up slime. Fungus gnats can also arrive in unpasteurised potting mix.
Do gnats bite?
Fungus gnat adults don't bite people or damage plants (UC IPM). Their larvae feed mainly on fungi and organic matter but can also chew roots (UC IPM).
What kills gnat larvae naturally?
Drying the soil, repotting in fresh pasteurised mix and biological products such as Bti or beneficial nematodes (UC IPM).
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.