How many grams is a quarter cup of self-raising flour?

A quarter cup of self-raising flour (self-rising flour) weighs

31g

That's 1.1oz. In a 250ml metric cup it's 33g.

This uses the US cup, the size American recipes mean. A quarter cup is 59ml by volume.

USDA measures American self-rising flour, which has salt added. UK self-raising flour weighs the same per cup but contains no salt.

Other amounts of self-raising flour

Where these numbers come from

USDA FoodData Central, SR Legacy, food 20082: “Wheat flour, white, all-purpose, self-rising, enriched”. USDA lists these household measures for it:

1 cup = 125g

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.