Explainer

Can ChatGPT convert a recipe to metric? We tested GPT-6 Astra against King Arthur's chart

Published October 10, 2026 Last checked October 10, 2026

The answer from one real test: yes, and better than we expected

We gave OpenAI’s GPT-6 Astra — the model OpenRouter lists as OpenAI’s flagship, slug openai/gpt-6-astra — a US-volume baking recipe written in cups, a stick of butter, teaspoons, °F, and an 8-inch pan. We asked for a complete metric conversion: grams, Celsius, and centimetres, with the full method rewritten.

It did the job well. Of the ten ingredient weights we checked against King Arthur Baking’s ingredient weight chart, nine matched to within about a gram or two. The oven temperature was converted correctly (350°F is 176.7°C; it gave 175°C and labeled it as a conventional, non-fan setting). The pan, the parchment overhang, the toothpick position, and the bar size in the method were all converted to centimetres. The ingredient list and method stayed consistent.

The one meaningful miss was packed brown sugar: 220 g instead of King Arthur’s 213 g — about 3% heavy. In blondies that’s unlikely to ruin anything, but it shows the model was using a generic figure rather than a specific reference, which is exactly where conversions drift in recipes that are less forgiving.

This was an OpenRouter API output test on October 10, 2026. We did not bake the recipe. One prompt and one model is a small sample, and the ChatGPT consumer app may use a different model, system instructions, or tools, so it won’t necessarily answer the same way. The full prompt and raw output are saved in our research notes.

The recipe we gave it

Brown sugar chocolate chip blondies for one 8-inch square pan, 16 bars:

• 1/2 cup (1 stick) unsalted butter, melted
• 1 cup packed light brown sugar
• 1/4 cup granulated sugar
• 1 large egg
• 1 teaspoon vanilla extract
• 1 1/4 cups all-purpose flour
• 1/2 teaspoon baking powder
• 1/2 teaspoon table salt
• 3/4 cup semisweet chocolate chips
• 1/2 cup chopped walnuts

The method used 350°F, an 8-inch square metal pan lined with a 1-inch parchment overhang, a 25–30 minute bake checked with a toothpick 1 inch from the edge, and 16 bars of about 2 inches square.

We chose this recipe because it mixes the conversions that commonly go wrong: flour (where cup weights vary widely), packed brown sugar, butter in sticks, small teaspoon amounts, chunky add-ins, °F, and inch measurements buried inside the method rather than in the ingredient list.

The prompt:

Convert this recipe completely to metric. Give every ingredient in grams (or millilitres where that is more natural), the oven temperature in Celsius, and the pan size in centimetres. Return the complete converted recipe — ingredient list and numbered method — and briefly list any assumptions you made.

The audit: every number checked

Reference weights are from King Arthur’s chart: all-purpose flour 120 g per cup, packed brown sugar 213 g per cup, butter 113 g per 1/2 cup, granulated sugar 198 g per cup, chocolate chips 170 g per cup, chopped walnuts 113 g per cup, baking powder 4 g per teaspoon, table salt 18 g per tablespoon, vanilla 14 g per tablespoon, and a large egg 50 g.

Two small additions weren’t in the original: it suggested about 5 g of extra butter for greasing the pan, and it described the egg as “50 g beaten egg.” Both are harmless. It also listed its assumptions, including that the 176.7°C was rounded to 175°C and the 20.3 cm pan to a standard 20 cm one.

Is the °F to °C conversion right?

Yes. The formula is (°F − 32) × 5 ÷ 9, so 350°F is 176.7°C. Recipes conventionally round this to 175°C or 180°C. What matters more is that GPT-6 Astra said conventional/non-fan. If you have a fan (convection) oven, you usually set it lower than the conventional temperature; follow your oven’s manual or the recipe’s stated fan setting, and check early. We cover the fan adjustment in how to convert cups to grams and °F to °C.

Does rounding 8 inches to 20 cm matter?

Slightly. An 8-inch square is about 20.3 cm per side, so a 20 cm pan has roughly 3% less area and the batter sits a touch deeper. For blondies that’s within normal variation, but it’s a reason to rely on the toothpick test rather than the clock. For bigger pan changes, see how to convert a recipe for a different pan size.

Where ChatGPT metric conversions still go wrong

This test went well, but a clean result on one recipe doesn’t make every conversion trustworthy. The brown sugar miss points to the general risk: a cup is a volume and a gram is a mass, and the right number depends on the ingredient and how it’s measured. These are the places to check any AI conversion, from this model or another:

• Flour. Published cup weights for all-purpose flour vary between sources. If a converted recipe gives a flour figure, check it against the source the recipe was written for, or a named chart.
• Packed vs. unpacked. Brown sugar, and anything “packed” or “lightly spooned,” depends on technique. This is where our test drifted.
• Butter in sticks. One US stick is 1/2 cup and 113 g. A model that converts “1 stick” by a generic volume figure can land off.
• Tiny amounts. Gram figures for 1/4 teaspoon of salt or yeast are small enough that a kitchen scale may not read them accurately. Keeping teaspoons (or 5 mL measuring spoons) for small amounts is often more practical.
• Numbers inside the method. Pan sizes, “1 inch from the edge,” and “cut into 2-inch bars” are easy to miss. GPT-6 Astra caught all of them here; check that yours did.
• Fan vs. conventional oven. A Celsius number with no oven type is incomplete.

For a full chart of common ingredient weights, see our cups to grams chart for baking.

How to ask for a better metric conversion

Our prompt was short, and it worked. If you want to reduce the chance of drift, add a reference and a consistency check:

Convert this recipe completely to metric using King Arthur Baking’s ingredient weight chart. Give grams for all dry ingredients and butter, Celsius for a conventional oven and the fan equivalent, and centimetres for every measurement in the method. Keep teaspoons for amounts under 5 g. List the gram figure you used per cup for each ingredient so I can check it.

Asking it to show the per-cup figure makes the audit take a minute instead of ten.

A two-minute check for any converted recipe

1. Check flour and every sugar against a named weight chart.
2. Confirm butter in sticks became 113 g per stick.
3. Recalculate the oven temperature and check it says fan or conventional.
4. Look for inches, °F, or cups left in the method.
5. Check the pan size and expect to rely on a doneness test.
6. Make sure the method still refers to every ingredient by its new amount.

Converting recipes in Sous Chef

ChatGPT is fine for a one-off conversion. The friction comes afterwards: you now have a US version in a browser tab and a metric version in a chat, and next month you want to scale it or swap an ingredient.

In Sous Chef on iPhone, metric/US is a setting on the recipe, not a rewrite: after you import a recipe from a link, pasted text, or photos, you can flip the whole recipe between US and metric at once and flip it back. Other edits — scaling, substitutions, dietary changes — are tracked separately on a working copy, with edited lines marked and the original kept for undo. It also works inside ChatGPT and Claude through the Sous Chef connector if you’d rather start in a chat. It is free to download and try; heavier AI use may need a subscription later.

Any converter, including Sous Chef, is making ingredient-weight assumptions. For baking you care about, check the key ingredients against a weight chart, as above. Sous Chef is iPhone only and has no nutrition tracking or meal planning. If you just want a single number, a printed weight chart on the fridge is quicker than any app.

Common questions

Can ChatGPT convert cups to grams accurately?

In our test of GPT-6 Astra, most cup-to-gram conversions matched King Arthur’s chart, but packed brown sugar came out 3% heavy. It’s a good first draft; check flour and sugars against a named weight chart before baking.

Did ChatGPT convert 350°F to Celsius correctly?

Yes. It gave 175°C and labeled it conventional (non-fan); the exact figure is 176.7°C, and 175°C or 180°C are both standard roundings. Use a lower setting for a fan oven per your oven’s manual.

What is 8 inches in centimetres for a baking pan?

About 20.3 cm. Recipes and pan makers usually call it a 20 cm pan, which has roughly 3% less area, so check doneness rather than relying on the clock.

How many grams is 1 cup of packed brown sugar?

King Arthur Baking’s chart lists 213 g for 1 cup of packed light or dark brown sugar. GPT-6 Astra gave 220 g in our test.

Is the ChatGPT app the same as this API test?

Not necessarily. We called openai/gpt-6-astra through the OpenRouter API on October 10, 2026. The ChatGPT app may route to a different model or use different instructions and tools.

Was the converted recipe baked?

No. This was an output audit against King Arthur’s published weights and standard temperature conversion. No recipe was baked.