How many cups is 100g of roasted chestnuts?

100g of roasted chestnuts is

⅔ cup + 1 tbsp

That's 0.70 US cups, 0.66 metric cups, or 11 level tablespoons.

Worked out from 1 US cup = 143g. Cups are rounded to the nearest kitchen fraction, with what's left over in level tablespoons. If you have scales, weighing is more accurate.

Other amounts of roasted chestnuts

200g: 1⅓ cups + 1 tbsp
250g: 1¾ cups
1 cup: 143g
Half a cup: 72g
A quarter cup: 36g
1 tablespoon: 8.9g
Full table for roasted chestnuts

Where these numbers come from

USDA FoodData Central, SR Legacy, food 12167: “Nuts, chestnuts, european, roasted”. USDA lists these household measures for it:

1 cup = 143g
1 oz = 28.35g
10 kernels = 84g

Fractions of a cup are the cup weight scaled down. The tablespoon figure is the cup weight divided by 16, since USDA doesn't list a tablespoon for this ingredient. The teaspoon is a third of the tablespoon. Metric-cup figures assume a 250ml cup, and Australian tablespoons hold 20ml. Weights under 10g are rounded to 0.1g, everything else to the nearest gram. Full source list.