💡 Grow Light Duration Calculator

A general reference for how many hours per day to run supplemental grow lights based on your plant's light needs, plus a suggested on/off schedule if you set a timer.

Plant light need

Lights on time

How the schedule is built

Each light-need category carries one fixed number of hours, and the off time is simply the on time plus that many hours, wrapped around the clock:

low-light   = 10 hrs    medium = 13 hrs    high / flowering = 15 hrs
lights off  = (lights on + hours) mod 24    dark period = 24 − hours

Switch on at 07:00 on the medium setting and the lights go off at 20:00, leaving 11 hours of dark. The high setting from the same start runs to 22:00 with 9 hours dark. A late start rolls over midnight: 20:00 plus 15 hours reads 11:00, meaning 11 the following morning.

What the hours do and do not tell you

Duration is only half the picture. What a plant banks each day is intensity multiplied by time — the daily light integral. A weak bulb a metre away for 15 hours can deliver less than a strong one at 30 cm for 10.
The dark period is not slack time. Respiration and, in short-day plants, flowering both depend on it. Poinsettia and Christmas cactus set buds only after weeks of long, unbroken darkness, so a lamp left on overrides them.
Count only supplemental hours if the plant also gets daylight from a window.

These three figures are broad category defaults, not readings from your room. Distance to the fixture, bulb output, reflectivity and how much daylight the window already supplies all shift the right answer, and species vary widely inside each band.

Frequently asked questions

How many hours a day should a grow light be on?

This tool uses 10 hours for low-light foliage plants, 13 for medium and 15 for high-light or flowering ones. Seedlings and herbs usually sit at the top of that range; a pothos in a dim corner sits at the bottom.

Can I leave a grow light on 24 hours?

Not sensibly. Most houseplants need a dark period for normal respiration, and round-the-clock light stops short-day plants flowering at all. The schedule always leaves at least 9 hours off.

Does it matter what time of day the light runs?

Less than the total. Plants respond to the length of the light and dark blocks rather than the clock, so pick times that suit you and keep them consistent rather than shifting the cycle around.