How Many Oxygen Absorbers Do I Need? The 21% Rule and a Size Chart

You need roughly the air left in the container, in milliliters, times 0.21, plus a margin. Air is about 21% oxygen, and an absorber rated at 300 cc can take up about 300 ml of oxygen. For a 1-gallon bag, that works out to about 350 cc for rice, 600 cc for pasta or beans and 800 cc for freeze-dried food, with a 25% margin.

This guide explains the rule, gives a chart for common bag sizes and includes a calculator for your own containers.

What is the 21% rule?

Oxygen absorbers only need to remove the oxygen, and oxygen is about 21% of the air. The other 79% is mostly nitrogen, which does not harm dry food. So the question is not how big the bag is, but how much air stays inside once it is full.

The formula is:

Absorber (cc) ≈ container volume (ml) × share of air × 0.21 × (1 + margin)

The share of air depends on the food. Fine, dense food like rice or flour packs tightly and leaves little air. Pasta, beans and oats leave gaps. Freeze-dried food and fruit chips are mostly air. The method follows the approach described by Sorbent Systems, measuring volume of space, a common reference for absorber sizing.

How do I calculate it for my container?

Enter the volume, food type, number of containers and margin below. The calculator shows the air and oxygen left inside, the cc per container and how many 300, 500 or 2,000 cc absorbers that means.

It uses three food types: dense (35% air), medium (60% air) and light (80% air). The margin defaults to 25%. Always round up to the next absorber.

How many oxygen absorbers per bag size? (Chart)

Use this chart as a starting point; it applies the 21% rule with a 25% margin. Volumes are the nominal US sizes. The oxygen figures are before the margin.

Container	Volume	Dense food (rice, flour)	Medium (pasta, beans, oats)	Light (freeze-dried, chips)
½ pint bag	0.24 L	17 ml O₂ → 22 cc	30 ml → 38 cc	40 ml → 50 cc
1 quart bag or jar	0.95 L	70 ml → 87 cc	119 ml → 149 cc	159 ml → 199 cc
½ gallon bag	1.9 L	139 ml → 174 cc	238 ml → 298 cc	318 ml → 397 cc
1 gallon bag	3.8 L	278 ml → 348 cc	477 ml → 597 cc	636 ml → 795 cc
5 gallon bag	18.9 L	1,391 ml → 1,740 cc	2,385 ml → 2,982 cc	3,180 ml → 3,976 cc

In practice, with common absorber sizes:

Container	Dense	Medium	Light
½ pint	one 100-300 cc	one 100-300 cc	one 100-300 cc
1 quart	one 300 cc	one 300 cc	one 300 cc
½ gallon	one 300 cc	one 300 cc (little margin) or one 400 cc	one 400 cc or two 300 cc
1 gallon	one 400 cc	two 300-400 cc	two 400-500 cc
5 gallon	one 2,000 cc	two 2,000 cc	two 2,000 cc

These are estimates. A bag filled to the top holds less air than one filled halfway, and packing styles vary.

What do the worked examples look like?

Here are three common cases, with the math shown.

A 1-gallon bag of white rice. 3.8 L = 3,800 ml. At 35% air, 3,800 × 0.35 = 1,330 ml of air. Oxygen: 1,330 × 0.21 ≈ 279 ml. With 25% margin: about 349 cc. One 400 cc absorber covers it.

A 1-gallon bag of pasta. 3,800 × 0.60 = 2,280 ml of air. Oxygen: 2,280 × 0.21 ≈ 479 ml. With margin: about 599 cc. Use two 300 cc or two 400 cc absorbers.

A 5-gallon bucket liner of dry beans. 18,930 ml × 0.60 ≈ 11,360 ml of air. Oxygen: about 2,385 ml. With margin: about 2,980 cc. Use two 2,000 cc absorbers, or eight 400 cc ones.

The rule of thumb of 300-500 cc per gallon bag sits between the rice and pasta cases. For airy foods, go higher.

Are the absorbers in kits big enough?

For quart bags, yes; for gallon bags of light food, often not. Here is how the kits in our catalog line up:

Kit or pack	Absorbers	Per gallon bag of rice	Per gallon bag of pasta
ZRLEI refill pack	100 × 400 cc	1	2
ROYLO GLOBE 120 kit	100 × 400 cc + 50 × 500 cc	1 × 400 cc	2 × 400 cc, or 1 × 500 cc with little margin
Kits with 300 cc absorbers	300 cc each	1 (little margin)	2

A refill pack like the ZRLEI 400 cc absorbers fills the gap when a kit falls short. The ROYLO GLOBE kit has the most spare capacity of the kits, with 150 absorbers for 120 bags. Compare all options in oxygen absorbers.

How do I handle absorbers so they keep their capacity?

Open them last, use them within 15-30 minutes and reseal the rest immediately. Absorbers start reacting as soon as their pack meets air, and every minute on the counter uses up capacity.

1. Fill and prepare all your bags first.
2. Open one pack and take only what you need.
3. Reseal the spares in an airtight glass jar or a small heat-sealed bag.
4. Seal each bag right after adding its absorber.
5. Check after a day or two. A well-sealed bag often pulls in slightly, because about a fifth of the air is gone.

What mistakes should I avoid?

Sizing by bag volume alone, using absorbers that sat out and trusting a single absorber for every size.

• Ignoring food density. A gallon of pasta needs almost twice the absorber of a gallon of rice.
• Rounding down. Too little absorber leaves oxygen behind. Too much does no harm.
• Reusing old absorbers. An absorber that has already reacted cannot be recharged.
• Skipping the check. Look at each bag after a day or two; a bag that stays loose and puffy may have a weak seal.

For the complete packing process, read long-term food storage step by step. For bags, see the best mylar bags and oxygen absorbers.