What Is the 3/8 Rule for Pizza?
The 3/8 rule states that when you increase a pizza's diameter by 3/8 (37.5%), the total area of the pizza roughly doubles.
For example, increasing an 8" pizza by 3/8 gives you an 11" pizza — which has approximately twice the surface area (and therefore twice the amount of pizza). This happens because pizza area is calculated using πr² — area scales with the square of the radius, not linearly with diameter.
This rule is critical for pizza shop owners because it means your larger pizzas contain far more food than customers (and many owners) realise. A 14" pizza is not 56% bigger than a 9" pizza — it is 142% bigger (2.42x the area). If your pricing does not reflect this, you are losing money on every large pizza you sell.
The Maths Behind the 3/8 Rule
Pizza is a circle, and the area of a circle is calculated as π × r² (pi times the radius squared). The key word is “squared” — this means area grows much faster than diameter.
Worked example: 8" vs 11" pizza
- 8" pizza: Area = π × 4² = 50.3 sq inches
- 11" pizza: Area = π × 5.5² = 95.0 sq inches
- Increase: 11 is 37.5% (3/8) larger than 8, but the area is 89% larger — nearly double.
The rule is an approximation — the exact doubling factor is closer to 1.89x rather than a perfect 2x. But for practical pricing purposes, the 3/8 rule is a reliable mental shortcut: increase the diameter by about a third, and you are giving the customer roughly twice as much pizza.
Pizza Size, Area & Pricing Comparison
This table shows how dramatically area (and food cost) increases with size — and what your pricing should look like to maintain margins.
| Size | Area (sq in) | vs 9" | Base Price |
|---|---|---|---|
| 9" (Small) | 63.6 | 1x | £6.99 |
| 12" (Medium) | 113.1 | 1.78x | £9.99 |
| 14" (Large) | 153.9 | 2.42x | £12.99 |
| 18" (XXL) | 254.5 | 4x | £17.99 |
Example prices shown. Actual pricing depends on your food costs, market, and location.
Why Most Pizza Shops Under-Price Large Pizzas
These three common mistakes silently destroy margins on your most popular sizes.
Flat Topping Prices
Charging £1 for a topping on every size means you give away 4x the cheese on an 18" pizza for the same price as a 9". Your food cost on large pizzas silently eats your margin.
Linear Price Scaling
Doubling the diameter from 9" to 18" quadruples the area. If your 9" is £6.99 and your 18" is only £13.99, you are selling 4x the pizza for just 2x the price.
No Size-Based POS Logic
Without automatic size-based pricing in your POS, staff must manually calculate or memorise different topping prices per size. They forget, they round down, and margins leak every shift.
How a Pizza POS Fixes Your Pricing
A pizza-specific POS with automatic size-based pricing applies the 3/8 rule for you. When you set up your menu, you define base prices and topping prices per size. The POS then calculates the correct total automatically — staff never need to think about it.
Example: Pepperoni Pizza
9" Small
£6.99
Base £5.99 + pepperoni £1.00
14" Large
£14.49
Base £12.49 + pepperoni £2.00
18" XXL
£19.99
Base £17.49 + pepperoni £2.50
With Posso, these prices are calculated automatically when staff or customers select a size. No manual maths, no errors.
Posso's pizza builder goes further: when a customer selects a 14" pizza and adds extra mozzarella, the topping price adjusts to the 14" rate automatically. Split-sided pizzas are priced correctly too — half toppings are charged at half the topping rate for that size.
The result? Your margins stay consistent whether a customer orders a 9" Margherita or an 18" fully loaded with extra toppings on each half. The 3/8 rule is baked into your pricing — automatically, on every order.
The Margin Impact of Getting This Right
Consider a pizza shop selling 200 large pizzas per week. If each large pizza is under-priced by just £1.50 (because toppings are not scaled by size), that is £300 per week in lost margin — or £15,600 per year.
Lost per pizza
-£1.50
Lost per week (200 pizzas)
-£300
Lost per year
-£15,600
A Posso POS starts from £499 + VAT. The size-based pricing feature alone can pay for the entire system within the first month. Self-order kiosks from £699 + VAT add further revenue by increasing average order value by 20–30%.
Get Size-Based Pricing for Your Pizza Shop
Pizza Sizing & Pricing — FAQs
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