18/6 vs 20/4 vs 24/0: The Best Veg Light Schedule
By The Budometry TeamPublished Updated
More light hours means more photosynthesis, but also more electricity and less recovery time. For most home growers the answer is 18/6, and here is why.
Part of our pillar guide: Cannabis Grow Lights: Sizing, Height and Schedules
18/6 — the default
Eighteen hours on, six off gives near-maximum growth with a real dark period. Plants perform root work and respiration in the dark, and the six hours off drops your power bill by a quarter versus running around the clock.
20/4 — for autoflowers in a hurry
Some auto growers see slightly faster growth on 20/4. The gain over 18/6 is small and the extra heat load in a small tent can cancel it out.
24/0 — rarely worth it
Constant light works but returns diminish sharply, heat builds, and the plant never rests. It also means a dehumidifier and fans running 24 hours with no cool-down window.
Flower is not negotiable
Photoperiod plants flower on 12/12 with a completely uninterrupted dark period. A light leak during those 12 hours can revert plants to veg or trigger hermaphrodite flowers.
Frequently asked questions
- Do I need a timer?
- Yes. Manual switching is the single most common cause of light-schedule stress. A dual-outlet digital timer costs about the price of a coffee run.
The full guide on this topic
Cannabis Grow Lights: Sizing, Height and Schedules
How to size an LED to your tent, set hanging height by stage, run 18/6 and 12/12 on a timer, and hit the PPFD and DLI numbers that actually grow flower.