Do I really need to weigh everything?

Weigh for the first three or four batches, then stop. The point of weighing is to develop a sense of what 30:1 looks like in your own materials. After that, volume estimates with occasional weighing are accurate enough, and the pile itself is the final arbiter.

What is the single most common mistake?

Using far more browns than intended because they look wet. Grass clippings and kitchen waste are 75-85 % water and are visually obvious; leaves at 25 %, straw at 14 % and cardboard at 8 % are drier than they appear. Batch logs that get recorded as 'won't heat' are very often 60:1 piles that were intended to be 30:1.

Method · wire bay · posted 10 Sep 2026

Working out C:N and moisture before the pile is built

Every smell problem in this log traces back to a ratio that was wrong before the material went in. The arithmetic takes ten minutes, needs nothing but a scale, and prevents most of what goes wrong afterwards.

posted 10 Sep 2026 · By The Compost Desk · 5 turns · 11 core readings

01The sheet

Feedstock as charged, with the carbon accounted row by row
material	kg in	C:N	water	role	carbon
Example: dry autumn leaves	10.0	50:1	25%	brown	9.8
Example: fresh grass clippings	10.0	17:1	78%	green	9.4
Example: kitchen vegetable waste	5.0	17:1	80%	green	4.7
Example: shredded cardboard	4.0	350:1	8%	brown	4.0
Blended	29.0	27:1	50%	28.0 kg C-eq

10:1 120:1 27:1

Blended C:N as charged 27:1 · target 30:1 · moisture 50%

02Batch facts

Bin: wire bay
Thermal class: Thermophilic (crossed 55 °C)
Turns: 5
Days to finish: 0
Target C:N: 30:1 (as charged 27:1)
Moisture at build: 55 % (as charged 50 %)
Odour risk: low
Space needed: No space needed — a kitchen scale, a notebook and about ten minutes per batch before it is built.
Time to finish: Ten minutes of arithmetic before you build, which is what decides whether the next 90 days go well.

Raw feed4 materials, 29.0 kg charged

Hot corepeak 61 °C (day 10), held 55 °C for 3 consecutive readings

CuredTen minutes of arithmetic before you build, which is what decides whether the next 90 days go well

Cross-section: wire bay — Section through the bin

03The carbon and nitrogen arithmetic, worked once

Take the example above: 10 kg of leaves at 50:1, 10 kg of grass at 17:1, 5 kg of kitchen waste at 17:1, 4 kg of cardboard at 350:1. Carbon in the leaves is 10,000 × (50/51) = 9,804 g and nitrogen is 10,000/51 = 196 g. Grass: 10,000 × (17/18) = 9,444 g C, 556 g N. Kitchen waste: 5,000 × (17/18) = 4,722 g C, 278 g N. Cardboard: 4,000 × (350/351) = 3,988 g C, 11 g N. Totals: 27,958 g C against 1,041 g N, or 26.9:1. That is a good starting ratio for a hot pile. Note how much nitrogen the cardboard contributes in weight terms — 4 kg of it, and almost no nitrogen at all.

04The four reference numbers worth memorising

Nothing here needs a spreadsheet. Four figures cover most garden material: grass clippings 17, kitchen vegetable waste 17, horse or goat manure with bedding 25-30, autumn leaves 50. Add cardboard at 350, sawdust at 450, straw at 70-80 and coffee grounds at 20 if you use them. A pile built from equal weights of material averaging 50 and material averaging 17 lands near 33, which is why the practical rule of thumb — two parts brown to one part green by volume — works as often as it does.

05Moisture is a weighted average, not a feeling

The same arithmetic applies to water. In the example: 10 kg of leaves at 25 % contributes 2.5 kg of water, 10 kg of grass at 78 % contributes 7.8 kg, 5 kg of kitchen waste at 80 % contributes 4.0 kg, and 4 kg of cardboard at 8 % contributes 0.3 kg. Total water 14.6 kg against a total mass of 29 kg, so 50 % — already in range, no water needed. Most first-time batches are built too dry because the browns look wetter than they are; a 25 % moisture leaf pile feels damp to the hand but contributes almost nothing.

06When the numbers and reality disagree

If the pile is at 30:1 and 55 % and still will not heat, the number to question is not the ratio but the piece size and the mixing. Material chopped at 10 cm behaves like a much higher C:N than the same material chopped at 3 cm, because the microbes cannot reach the interior. Likewise a pile that is layered rather than mixed has local zones far outside the average. The instrument to reach for at that point is a fork, not a calculator.

07Turn schedule

day -1 Weigh each material on a bathroom scale in a bucket, subtracting the bucket weight. Estimate C:N from the reference tables rather than guessing.curve 20 °C · counted from day 0
day -1 Compute total carbon and total nitrogen, then the ratio. If it is above 35, add greens; below 22, add browns.curve 20 °C · counted from day 0
day -1 Compute the moisture weighted average. Target 50-60 %; add water in 2 L passes with mixing between.curve 20 °C · counted from day 0
day 0 Build the pile and record the numbers in the logbook, including what you actually added versus what you planned.curve 20 °C · counted from day 0
day 4 Verify: a pile built at 30:1 and 55 % moisture should pass 50 °C within four days. If it has not, recheck the numbers before adding anything.curve 52 °C · counted from day 0

08Temperature curve, as it ran

Temperature curve: Working out C:N and moisture before the pile is built — Readings taken 30 cm into the core before and after each turn. Squares mark turn days; the dashed line is 55 °C.

day 0–2Phases · mesophilicday 4Phases · warmingday 7–14Phases · thermophilicday 18–30Phases · warmingday 40–50Phases · mesophilic

Smell notes

This entry has no batch to smell, but the correlation is worth stating: of the eleven smell complaints recorded across the batches in this log, nine occurred in batches whose C:N or moisture was outside the target band at build time. The two exceptions were both caused by compaction of material that was measured correctly but not mixed thoroughly. Ratios are not a guarantee, but they are the cheapest insurance available.

09Questions this batch raised

Do I really need to weigh everything?: Weigh for the first three or four batches, then stop. The point of weighing is to develop a sense of what 30:1 looks like in your own materials. After that, volume estimates with occasional weighing are accurate enough, and the pile itself is the final arbiter.
What is the single most common mistake?: Using far more browns than intended because they look wet. Grass clippings and kitchen waste are 75-85 % water and are visually obvious; leaves at 25 %, straw at 14 % and cardboard at 8 % are drier than they appear. Batch logs that get recorded as 'won't heat' are very often 60:1 piles that were intended to be 30:1.