Powdered or confectioner’s sugar is often sold in 1 or 2 pound bags or boxes (=16 or 32 oz), but most recipes typically use smaller amounts in varying units of measurement. Below is a table to convert between ounces, cups and pounds, but read on for some caveats on why it isn’t 100% precise.
|Initial amount:||1 ounce||1 cup||1 pound||2 pounds|
|..is this many ounces||1||4.673||16||32|
|..is this many cups||0.214||1||3.424||6.847|
|..is this many pounds||1/16||0.292||1||2|
According to the Food and Agriculture Organization of the United Nations’ International Network of Food Data Systems Density Database Version 2.0 (a mouthful, but at least you know you can trust it), powdered sugar has a bulk density of 0.56 grams (this was reportedly truncated to 2 decimal places) per milliliter at a temperature of 4 degrees Celsius (or 39.2 degrees Fahrenheit). This was used to calculate the values in the table above, but two things can affect it: moisture and temperature.
Density will decrease as temperature goes up, but for practical purposes this can be ignored since it only involves changes of a fraction of a percent – even across temperature differences of over 100 degrees. This small change becomes further irrelevant if you measure using weight (ounces, grams, etc.) instead of volume (spoons, cups, etc.) since a change in density won’t affect mass.
Moisture and humidity can be a much bigger problem. When contaminated with water, you will get less powdered sugar for the same amount of weight (which makes sense since some of the weight is now water) and conversely more for the same amount of volume (this is a little less intuitive). Why does the density increase? While dry powdered sugar has a density of 0.56 grams per milliliter this is its “bulk density” which includes the microscopic spaces of air inside it (like the empty space in a jar of marbles). Water collapses this (using the marble analogy it would be like melting them) to bring it closer to the density of solid, crystalline sucrose of 1.59 grams per milliliter at 68 degrees Fahrenheit (source: National Institutes of Health PubChem database).
This is another reason why sifting is necessary apart from removing clumps. So how do you prevent clumps from forming in the first place? The USDA-published book Weight and Polarization Changes of Puerto Rican Raw Sugar in Storage and Shipment written by Robert Grant Martin in 1958 provides some storage advice derived from industry papers and handbooks (it looks at granular rather than powdered sugar, but the same principles apply):
- Store at below 75% relative humidity (moisture absorption goes from minimal below this and increases rapidly past this point) – lower is better with 50% relative humidity guaranteeing zero moisture absorption
- Even in humid climates, storage in a sealed airtight place prevents any more moisture getting in
- Heat can be used to lower relative humidity as hot air can hold more water vapor without it condensing