Lentils: every method

Lentils: cooked weight by method, from 100, 150 and 200 g raw
Method	Yield	100 g raw	150 g raw	200 g raw
Boiled, no salt	302.2%	302 g	453 g	604 g

Lentils: energy and protein per 100 g, raw and cooked, with sources
Method	Raw, per 100 g	Cooked, per 100 g	Source
Boiled, no salt	352 kcal · 24.6 g protein	116 kcal · 9 g protein	derived from dry matter · FDC 172420, FDC 172421

Dry lentils boil up to about 302.2% of their weight, roughly three times: 100 g dry becomes about 302 g, and a 60 g portion becomes 181 g. Among the foods in the converter, only couscous and quinoa gain more.

Why lentils triple

A dry lentil is only 8.3 g of water per 100 g. Boiled, USDA's lentils hold 69.6 g, so most of what you eat is water taken up in the pot. Nothing but water goes in, so the dry part stays the same, and the yield follows from the water alone: (100 − 8.26) ÷ (100 − 69.64) = 3.02.

That makes lentils sensitive to how you cook them. Lentils simmered until soft, or left in their liquid, keep taking up water; lentils drained while still firm weigh less. USDA's figure is for its boiled samples, without salt, and the entry doesn't say how long they cooked.

A worked example

Boil 60 g of dry lentils and expect about 181 g. The dry entry gives 211 kcal and 15 g of protein; the boiled entry, for 181 g, gives 210 kcal and 16 g. The energy matches almost exactly. The protein is about 10% higher by the boiled entry, a sign that USDA's raw and boiled lentils came from different samples rather than that cooking added any.

Should I weigh lentils dry or cooked?

Dry, when you cook them yourself: one weighing, before the water goes in, and the dry entry. If you cook a big pot for the week, weigh it once when it's done, without the pot, and divide by the dry weight to get your own yield; it will fit your lentils better than any table. In a soup the lentils share the pot with everything else, and the recipe-portion calculator is the better tool.

What about red lentils?

USDA has a raw entry for red lentils but no cooked one, so there is no red-lentil pair to derive a yield from. The converter uses USDA's pair for whole lentils.

Sources

USDA FoodData Central, SR Legacy: lentils, raw, 172420 and boiled without salt, 172421; red lentils, raw, 174284.
The yield is derived from the pair's water content; the USDA Table of Cooking Yields for Meat and Poultry covers meat and poultry only.

Chickpeas and beans are in the raw to cooked weight converter; for another staple that swells, see rice.