When you add water to dry foods like pasta, rice, oats, soup mixes, or protein powder, the carbohydrate content per 100g changes — even though the water itself adds zero carbs. This calculator shows you exactly how many carbs are in 100g of the prepared food after hydration.
Simply enter the dry weight, the carbs per 100g listed on the label, and how much water you add (or the final prepared weight). The tool also calculates total carbs in the whole batch and carbs per serving if you enter a serving size. Great for meal prep, carb tracking, and anyone following a low-carb or diabetes-friendly plan.
Whether you're counting macros, managing blood sugar, or just curious how cooking changes nutrition, this tool turns label data into practical per-serving numbers.
🥗 Quick Food Presets
Enter your food details and click Calculate.
How to Use This Calculator
This calculator is designed for any dry food that you hydrate before eating. To get started, follow these steps:
- Check the nutrition label on the package and note the carbohydrate value per 100g of the dry food.
- Weigh the dry food you plan to prepare and enter that number in the first field.
- Choose whether you want to work from the amount of water added or from the final prepared weight. If you measure water in millilitres, you can enter that number directly because 1 mL of water weighs 1 g.
- Optionally, enter a serving size in grams to see how many carbs are in the portion you actually eat.
Click Calculate to see the per-100g carb count of the prepared food, the total carbs in the whole batch, and a detailed step-by-step breakdown. Use Reset to clear the form and start again.
The Math Behind It
Water is the only ingredient being added, and water contains no carbohydrates. Therefore the total grams of carbohydrate in your batch remain exactly the same after hydration. What changes is the total weight, so the same carbs are spread over a larger mass.
Total carbs (g) = (Dry carbs per 100g ÷ 100) × Dry weight
Final weight (g) = Dry weight + Water added
Carbs per 100g prepared = (Total carbs ÷ Final weight) × 100
The optional serving carb value is then: (Carbs per 100g prepared ÷ 100) × Serving size. This simple dilution math is the same principle used by dietitians when calculating the nutrient content of rehydrated foods.
Worked Examples
Example 1: Cooked pasta. You weigh 80g of dry pasta with 72g carbs per 100g. First, find total carbs: (72 ÷ 100) × 80 = 57.6g. You cook it in 120g of water, so the final weight is 80 + 120 = 200g. The prepared pasta has (57.6 ÷ 200) × 100 = 28.8g carbs per 100g.
Example 2: Mashed potatoes. A 60g serving of dry potato flakes contains 80g carbs per 100g, so total carbs are 48g. You add 240g of boiling water. The final weight is 300g. The prepared mash contains (48 ÷ 300) × 100 = 16g carbs per 100g. If your serving is 150g, it contains 24g of carbs.
These examples show how the same dry carb value can translate into very different per-100g numbers depending on how much water you add.
Real-World Applications
This calculation is useful for meal prep, macro counting, and blood-sugar management. People following low-carb or keto diets often track net carbs carefully, and knowing the diluted carb value helps them portion foods correctly. It is also valuable for athletes who need to hit precise carbohydrate targets, and for parents preparing instant cereals or soup mixes for children.
Standard Carb Values for Common Dry Foods
The values below are typical ranges from common food labels. They may vary by brand and product, so always use the label on your package when available.
| Food | Carbs per 100g (dry) | Typical water ratio |
|---|---|---|
| Dry pasta | 70–75 g | 1 : 1.5–2 |
| White rice | 78–82 g | 1 : 1.5–2 |
| Rolled oats | 66–68 g | 1 : 2–3 |
| Soup mix | 50–60 g | 1 : 4–6 |
| Mashed potato flakes | 78–82 g | 1 : 4–5 |
| Protein powder | 5–15 g | 1 : 8–10 |
Water ratios are approximate and depend on cooking method and personal preference. If you know your final cooked weight, use that value instead of relying on a ratio.
Tips for Accurate Results
- Weigh the dry food on a food scale rather than using volume measurements, because cup sizes vary greatly.
- Use the same unit for all weights. The calculator is set up for grams, but if you measure in ounces, convert to grams first.
- If you boil pasta or rice and drain water, the absorbed water is what matters, not the total water used in the pot. Weigh the final cooked food if you are unsure.
- For foods that lose water during cooking, the calculation direction reverses: the carb concentration will rise as moisture evaporates.
Common Mistakes to Avoid
Entering the label's prepared serving size as the dry carbs value. Many instant products list two nutrition panels: one for dry mix and one for prepared. Always use the dry per-100g value in this calculator.
Not accounting for added carb ingredients. If you add milk instead of water, honey, or a sauce, those extra carbs must be tracked separately because the calculator only accounts for plain water.
Ignoring cooking losses. Some water evaporates during simmering. If accuracy is critical, weigh the final product directly rather than calculating from the starting water amount.
Frequently Asked Questions
Does water really add zero carbs?
Yes. Plain tap water, bottled water, and filtered water contain no carbohydrates. The carb content per 100g drops simply because the same amount of carbs is spread across more total weight.
Why does the percentage on the label change after cooking?
The label shows the dry food's nutrition. Once water is absorbed, the weight increases but the carbs do not, so the per-100g concentration decreases. The total carbs in the batch remain unchanged.
Should I track dry or prepared carbs?
Use whichever matches your habits, but be consistent. Tracking dry weight is the most accurate because it eliminates water variability. Tracking prepared weight is more convenient for foods you cook in batches and portion out after cooking.
Can I use this calculator for vegetables?
Yes, for any food that absorbs water without other carb additions, such as dried beans after soaking, bulgur, couscous, or chia seeds.
Further Reading
For comprehensive nutrition data, use the USDA FoodData Central database at fdc.nal.usda.gov. For background on carbohydrate chemistry and how foods are analysed, see the Carbohydrate article on Wikipedia.