A recipe that works for one cup doesn't always work for two. The math says it should — double the coffee, double the water, same ratio. The cup says it doesn't, and there are reasons.
Why simple doubling breaks
When you double the coffee, you double the bed depth. Water now has more grounds to travel through, which means:
- More contact time per drop, even at the same flow rate.
- More resistance, so the brewer drains slower.
- More heat loss — the bigger bed pulls more heat out of the water.
Net effect: a recipe that runs 2:30 at 15g may run 3:30 at 30g. That extra minute of contact time often pushes the cup past the extraction sweet spot and into bitter territory.
What to actually adjust when scaling up
Three small changes make a doubled recipe behave like the original:
- Coarsen the grind one notch. Bigger bed = more resistance; coarser grind compensates. The bigger the bed, the more this matters.
- Raise the brewer one size if you have it. V60 01 → V60 02 for 30g. The bigger size has more bed surface, which counteracts depth.
- Pour faster. A heavier total water mass on a deeper bed needs to keep moving. Slow pours stall.
If you don't have a bigger brewer and you're doubling on the smaller one, consider doing two brews back-to-back and combining. The second brew can chase the first while it's still warm.
Scaling down
The reverse is usually easier — going from 18g to 12g is more forgiving than going 18g to 36g. The bed is shallower, water moves faster, so:
- Tighten the grind to keep contact time up.
- Slow your pour if the brewer is going to drain in 1:30 instead of 2:30.
- Watch heat loss more carefully. Less water cools faster on the small bed.
When the math really doesn't work
Some methods don't scale at all. Cold brew is the cleanest example: at 1:8, doubling works because contact time is the variable, not flow. But in a French press, doubling means a deeper plunger track and a much harder press. The cup gets siltier.
For pour-over, anything beyond a 2× scale (15g → 30g) starts asking for a different brewer rather than a stretched recipe. That's not a failure — it's why bigger Chemexes exist.
A starter table
| Original | 2× | What changes |
|---|---|---|
| 15g coffee, V60 01 | 30g, V60 02 | Bigger brewer, coarsen 1 notch |
| 18g, Kalita 155 | 36g, Kalita 185 | Bigger brewer, coarsen 1 notch |
| 12g AeroPress | 18g AeroPress | Same brewer, tighten 1 notch (less bed depth than expected because of cylinder shape) |
| 200g cold brew | 400g cold brew | Same recipe, 1:8 holds linearly |