1/3 cup of caster sugar weighs approximately 63.3 g. That figure comes from the published weight of a full cup — 190 g, spooned and levelled, King Arthur Baking — divided by three. All the gram figures on this page are approximate: cup weights vary with how the ingredient is packed, levelled or poured, and with the batch itself, so verify with a kitchen scale whenever the result matters, especially in baking.
Sugar does more than sweeten: it holds moisture, browns the crust and sets the structure, so the difference between a level third of a cup and a heaped one shows up in the finished bake. Fine-grained white sugar that dissolves faster than granulated sugar. It is used for meringues, cocktails, shortbread and custards., and a third of a cup is one of the most common amounts in those recipes, small enough for a half batch or a glaze, large enough to matter to the result.
Conversion table for caster sugar
| Amount | Grams (approx.) |
|---|---|
| 1/4 cup | 47.5 g |
| 1/3 cup | 63.3 g |
| 1/2 cup | 95 g |
| 2/3 cup | 126.7 g |
| 3/4 cup | 142.5 g |
| 1 cup | 190 g |
| 2 cups | 380 g |
| 3 cups | 570 g |
| 1 tablespoon | 11.9 g |
| 2 tablespoons | 23.8 g |
| 1 teaspoon | 4 g |
The table is built from a single number — 190 g per cup — multiplied by each amount. A third of a cup is also 5 tablespoons plus 1 teaspoon, which is handy when your 1/3-cup measure is in the dishwasher: five level tablespoons (59.4 g) plus one level teaspoon (4 g) add up to 63.3 g. Read every figure as approximate and check it on a scale when precision matters.
Why a third of a cup is an awkward amount to eyeball
A third of a cup sits in an awkward spot. It is too small to judge by eye in a large bowl and too large to ignore, and most measuring-cup sets include a 1/3-cup measure precisely because guessing it goes wrong so often. People commonly fill a 1/2-cup measure two thirds full, or a 1-cup measure one third full, and both tricks are unreliable: the eye is poor at judging a third of a volume, and the error is usually on the generous side. Weighing 63.3 g removes the judgement entirely.
There is a second reason the weight matters for caster sugar specifically. Fine-grained white sugar that dissolves faster than granulated sugar. That character changes how it behaves in the cup — how it settles, how it packs, how much air it holds — and therefore how much a cup actually weighs on any given day. The published per-cup figure of 190 g is the standardised value, measured spooned and levelled by King Arthur Baking, and dividing it by three is the honest way to get a third of a cup. Any recipe that asks for a third of a cup of caster sugar was developed with that standard measure in mind.
Worked examples: scaling a recipe up and down with a third of a cup
The most useful way to learn a conversion is to use it. The examples below start with a single third of a cup, then double it, then scale a full recipe around it, using the same per-cup weight of 190 g throughout. Every gram figure is the per-cup weight multiplied by the amount needed — nothing here comes from a different source or a guess.
Example 1
Sweetening a small batch of muffins. A muffin recipe for six muffins calls for 1/3 cup of caster sugar. Weigh 63.3 g. If you double the recipe for a full dozen, you need 2/3 cup, which is 126.7 g — and because sugar also holds moisture, weighing it keeps the doubled batch as moist as the original instead of accidentally sweeter and denser.
Example 2
A glaze or frosting that needs just a little sweetness. A glaze for a single loaf cake might call for 1/3 cup of caster sugar (63.3 g) dissolved or beaten into the base. If the glaze tastes flat, add a tablespoon at a time — each tablespoon is about 11.9 g — rather than another third of a cup, so you can stop before it turns cloying.
Example 3
Cutting sugar back by a third. A cake recipe calls for 1 cup of caster sugar (190 g). Reducing it by a third leaves 2/3 cup (126.7 g). That is a sensible first cut: the cake still browns and stays moist, while a bigger cut can leave it pale and dry. Weigh the reduced amount rather than estimating it in the cup, because the cup is where the error creeps in.
How the weight was measured: method and source
The per-cup weight of caster sugar is 190 g, measured spooned and levelled, according to King Arthur Baking. The method is part of the number, not a footnote: the same ingredient measured a different way weighs a different amount, and a recipe developed with spooned-and-levelled flour will not behave the same if the flour was scooped and packed. There is no competing published figure for caster sugar in the data used for this page, so the headline number is simply the published per-cup weight divided by three. It is still labelled approximate, because any cup measure depends on how the ingredient is packed, levelled or poured, and on the batch itself.
If you are converting a recipe from another country, remember that a US cup is 236.6 ml while a metric cup is 250 ml, so a metric third of a cup is a slightly larger volume. The gram figures here are tied to the US cup through the published per-cup weight, so use them with US cup measures, or weigh the ingredient directly and ignore the cup altogether — which is the most reliable approach of all.
Measuring caster sugar without a scale
Caster Sugar is measured spooned and levelled: spoon it loosely into a dry measuring cup until it heaps above the rim, then sweep a straight edge across the top. Never scoop the cup through the bag and never tap the cup to settle it — both pack the ingredient down and add grams you did not ask for. On a scale, 63.3 g is the target and the method stops mattering.
When you have no scale at all, use the proper dry or liquid measure for the ingredient, level it carefully, and accept that the result will be approximate. For caster sugar, the most common mistakes are packing a spooned ingredient, leaving air pockets in a scooped one, reading a liquid measure from above instead of at eye level, and using a coffee mug or teacup as a measuring cup — none of which holds a standard third of a cup. If a recipe has failed before and you suspect the measuring, the scale is the cheapest fix in the kitchen: a basic digital scale costs less than the ingredients of two ruined bakes.
A useful habit is to weigh the empty bowl first, press tare, and add the ingredient until the display reads the target. For 1/3 cup that target is 63.3 g; for 2/3 cup it is 126.7 g; for a full cup it is 190 g. Tare between ingredients and you can build a whole recipe in one bowl without washing a single measuring cup.
Storage and keeping caster sugar at its best
Airtight container, cool and dry, indefinitely. Keeping caster sugar in the conditions the data recommends matters for weighing as well as for flavour: an ingredient that has absorbed moisture from the air weighs more per cup than a dry one, and a dried-out ingredient weighs less. If your caster sugar has been open for months in a humid kitchen, the per-cup weight will have drifted from the published 190 g, and the scale — which measures what is actually in the bowl — is the only way to know what you really added.
Buy caster sugar in quantities you will use within its storage life, keep the container sealed, and label it with the date you opened it. That small discipline keeps both the flavour and the weight predictable, which is what a conversion table depends on.
When a third of a cup is the wrong amount to use
Not every recipe should be scaled by thirds. If a recipe calls for 1 cup of caster sugar and you want a smaller batch, halving to 1/2 cup (95 g) is usually safer than cutting to a third, because halving keeps the ratios of the other ingredients simple too. A third of a cup earns its place when the recipe itself asks for it, when you are correcting a batter a little at a time, or when you are halving a recipe that started at 2/3 cup.
The other case to watch is leavening and setting agents mixed into caster sugar — if the flour in question already contains raising agents, changing the flour amount changes the raising power as well. Measure the flour accurately and do not adjust the raising agents separately unless the recipe tells you to.
Frequently asked questions
How many grams is 1/3 cup of caster sugar?
1/3 cup of caster sugar weighs approximately 63.3 g. It is calculated from the published per-cup weight of 190 g (spooned and levelled, King Arthur Baking) divided by three. Treat it as approximate and verify with a kitchen scale when the recipe matters.
How many tablespoons is 1/3 cup of caster sugar?
1/3 cup equals 5 tablespoons plus 1 teaspoon. One tablespoon of caster sugar weighs about 11.9 g and one teaspoon about 4 g, so five tablespoons plus a teaspoon come to approximately 63.3 g. Use level spoon measures, not heaped ones.
Is 63.3 g exact for 1/3 cup of caster sugar?
No. It is the published per-cup weight divided by three, rounded to one decimal place, and every cup weight is approximate because it depends on how the ingredient is spooned and levelled and on the batch. A kitchen scale reading of the actual ingredient is the exact figure for that batch; 63.3 g is the best standard estimate.
Can I use a 1/2-cup measure to get 1/3 cup of caster sugar?
You can fill it about two thirds full, but it is unreliable — the eye judges a third of a volume poorly and the error is usually generous. Weighing 63.3 g on a scale, or using a proper 1/3-cup measure, is more accurate. If you must estimate, fill the 1/2-cup measure, level it, and remove roughly a third.
Does the weight change if my caster sugar is old or damp?
Yes. Airtight container, cool and dry, indefinitely. Moisture absorbed from the air adds weight per cup, so old or damp caster sugar can weigh more than the published 190 g per cup. Store it sealed and weigh what you use rather than trusting the volume alone.
What is 2/3 cup of caster sugar in grams?
2/3 cup of caster sugar weighs approximately 126.7 g — twice the 1/3-cup figure of 63.3 g. A full cup is 190 g and 1/4 cup is 47.5 g, all from the same per-cup weight of 190 g.