12/12 is 12 hours of light and 12 hours of darkness per day. Most photoperiod 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 depends entirely on intensity. At 900 µmol/m²/s, 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, and Peterswald and colleagues (2023) tested longer flowering photoperiods in medicinal cannabis, 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 works, and some cultivars flower at 14–15 hours. Test your cultivar; that is what Mythbusters trials are for.
