Three 8-inch Rounds to 9×13 Rectangle:
The Dramatic Downsizing

8"
8"
8"

Three tall layers

18.9 cups total

→
9×13"

Single thin layer

10.9 cups

×0.58

Use only 58% of ingredients

Major Size Difference!

From tall wedding-style cake to single sheet - big adjustments needed

Understanding This Major Conversion

Volume Reduction

Three 8" rounds:18.9 cups
Total height: 4.5-6 inches!
One 9×13":10.9 cups
Height: only 1.5-2 inches
42% less volume needed

Structure Changes

From:

Impressive 3-layer tower with double frosting layers

To:

Single flat sheet with top-only frosting

⚠️ Completely different cake experience

Essential Adjustments for Success

Temperature: Must Reduce to 325°F

The thinner batter in 9×13 will burn at 350°F. Here's why:

8" rounds at 350°F:

1.5-2" depth protects from heat

9×13" at 325°F:

Only 1" depth needs gentler heat

Time: Dramatically Shorter

If layers took 25-30 min → Sheet needs only 18-22 min

Start checking:
16 minutes
Usually done:
18-20 minutes
Maximum:
22 minutes

Recipe Math: The 58% Rule

Original (3 layers):

  • • 4½ cups flour → 2⅝ cups
  • • 6 eggs → 3½ eggs (use 4)
  • • 1½ cups butter → ⅞ cup
  • • 3 cups sugar → 1¾ cups

💡 Pro Tip:

This conversion is so dramatic, consider making a different recipe designed for 9×13 instead!

Calculate Your Conversion

Pan Converter

🤔 Consider This Alternative

Instead of scaling down a 3-layer recipe by 42%, many bakers prefer:

1️⃣
Make the full 3-layer recipe

Use 58% for the 9×13 and make cupcakes with the rest

2️⃣
Use a 2-layer recipe instead

Two 8" rounds (×0.87) is much closer to 9×13

3️⃣
Find a dedicated 9×13 recipe

Recipes designed for sheet pans work better than converted layer cakes

Common Problems with This Conversion

❌ Loss of the "Wow Factor"

A 3-layer cake is impressive and celebratory. A sheet cake... isn't.

Solution: Add decorative piping, fresh fruit, or chocolate shavings to elevate the presentation.

⚠️ Weird Ingredient Amounts

58% of 6 eggs = 3.48 eggs? 58% of 1½ cups = ⅞ cup?

Solution: Round to nearest practical measurements and accept slight variations.

💡 Different Moisture Needs

Layer cakes have frosting between layers adding moisture. Sheet cakes don't.

Solution: Add 2-3 tbsp sour cream or an extra egg yolk to boost moisture.

When This Conversion Makes Sense

✅ Good For:

  • • Testing a recipe before making full version
  • • Using up leftover 3-layer recipe portions
  • • When you need less cake
  • • Casual gatherings vs. celebrations

❌ Skip When:

  • • Making for special occasions
  • • Recipe has complex ratios
  • • You want the layer cake experience
  • • Ingredient scaling gets too weird

Better Alternative Conversions

Two 8" → 9×13"

Much easier conversion (×0.87)

Two 9" → 9×13"

Nearly perfect match (×0.92)

9×13" → Two 9"

Sheet to layers