# 12/12

> 12/12 is the classic flowering schedule for photoperiod cannabis: 12 hours of light and 12 hours of uninterrupted darkness, which triggers and sustains flowering.

- Canonical: https://genetixbay.com/knowledge/glossary/12-12.html
- Also written: 12-12, 12/12 light cycle, twelve twelve, 12 on 12 off, flip to 12/12, the flip
- Glossary category: Environment and light · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

12/12 is 12 hours of light and 12 hours of darkness per day. Most [photoperiod](https://genetixbay.com/knowledge/glossary/photoperiod.md) cannabis has a critical day length somewhere around 12–14 hours: shorter days (longer nights) trigger flowering. 12/12 sits safely on the short side for nearly every drug-type cultivar, so it became the default. "Flipping" means switching from a veg schedule to 12/12.

## In practice

Because the light period is fixed, flower [DLI](https://genetixbay.com/knowledge/glossary/dli.md) depends entirely on intensity. At 900 [µmol/m²/s](https://genetixbay.com/knowledge/glossary/ppfd.md), 12/12 gives about 39 mol/m²/day. Darkness must be truly dark: Eckels and Bugbee (2024) reported that even low-level night light can delay cannabis flowering.

12/12 is the safe choice, not the only one. Zhang and colleagues (2021) showed critical photoperiods vary widely between [hemp cultivars](https://genetixbay.com/knowledge/glossary/hemp.md), and Peterswald and colleagues (2023) tested longer flowering photoperiods in [medicinal cannabis](https://genetixbay.com/knowledge/glossary/medical-vs-adult-use.md), finding some cultivars flowered and yielded well on more than 12 hours.

## Units and formula

DLI = PPFD × 12 × 0.0036.

## Don't confuse with

- **13/11**: a longer flowering day, endorsed by GenetixBay for many cultivars.
- **Photoperiod**: the general term for day length.

## Myth check

"Flower must be exactly 12/12" is a *Myth*. GenetixBay's position: [13/11](https://genetixbay.com/knowledge/glossary/13-11.md) works, and some cultivars flower at 14–15 hours. Test your cultivar; that is what [Mythbusters](https://genetixbay.com/knowledge/mythbusters.md) trials are for.

## Related terms

[13/11](https://genetixbay.com/knowledge/glossary/13-11.md), [18/6](https://genetixbay.com/knowledge/glossary/18-6.md), [Photoperiod](https://genetixbay.com/knowledge/glossary/photoperiod.md), [Light leak](https://genetixbay.com/knowledge/glossary/light-leak.md), [DLI](https://genetixbay.com/knowledge/glossary/dli.md)

## Read more

- [Photoperiod and light leaks: 18/6, 12/12 and the dark period](https://genetixbay.com/knowledge/environment/photoperiod-light-leaks.md)
- [Photoperiod vs autoflower, and how autoflowering is inherited](https://genetixbay.com/knowledge/basics/photoperiod-vs-autoflower.md)

## Sources

1. Zhang M, Anderson SL, Brym ZT, Pearson BJ, 2021. Photoperiodic flowering response of essential oil, grain, and fiber hemp (Cannabis sativa L.) cultivars. Frontiers in Plant Science 12:694153. https://doi.org/10.3389/fpls.2021.694153
2. Peterswald TJ, Mieog JC, Azman Halimi R, et al., 2023. Moving away from 12:12; the effect of different photoperiods on biomass yield and cannabinoids in medicinal cannabis. Plants 12(5):1061. https://doi.org/10.3390/plants12051061
3. Eckels M, Bugbee B, 2024. Night light pollution delays flowering in soybean and cannabis (ASHS 2024 conference abstract). https://ashs2024.sched.com/event/1dtbo/ce-4-night-light-pollution-delays-flowering-in-soybean-and-cannabis