Why a 60cm Egg Tart Is Mostly Crust: Geometry of the Filling-to-Crust Ratio

On September 20, an internet-famous bakery (later identified as Bao Shifu) launched a 60cm long egg tart priced at 18.9 yuan, selling over 700,000 units in eight days. Customers complained that the tart was mostly layered puff pastry with only a thin layer of egg custard. The bakery acknowledged the feedback and promised optimization.

Modeling the Tart as a Long Trough

The article simplifies the tart into a rectangular trough: length L, width W, depth D, and crust thickness t. The filling volume is approximately W × D × L. The crust volume comprises the bottom ( W × L × t), two long side walls ( 2 × D × L × t), and two end caps ( 2 × W × D × t). The filling-to-crust ratio is:

Ratio = (W × D) / [t × (W + 2D) + 2W D t / L]

This is a variant of Galileo's square–cube law: crust scales with surface area, filling with volume. The ratio depends on the smallest dimensions.

Length Has Negligible Effect

Using example values W = 5 cm, D = 1.5 cm, the ratio changes only ~3% when length increases from 30cm to 60cm, and actually improves slightly because the end-cap term becomes negligible. Halving depth reduces the ratio by 38%; halving width reduces it by 27%. A round tart of diameter 7cm yields a similar ratio; a 5cm-wide rectangular tart has a side-wall term equivalent to a 10cm round tart, making the rectangular shape more crust-efficient.

Puff Pastry Expansion Eats the Narrowest Dimension

The model assumes fixed crust thickness t, but puff pastry expands during baking. If the pastry bulges inward by Δ, the effective cross-section shrinks from W × D to (W - 2Δ) × (D - Δ). For W = 5 cm, D = 1.5 cm, a 0.5cm bulge leaves only 44% of the original cross-section; if depth is only 1cm, just 40% remains. The expansion consumes the smallest dimension first.

Length Returns via Tilt During Filling

While length cancels out in the static ratio, it reappears during production. A tilt angle θ creates a liquid-level difference of L × sin θ between the two ends. For a 7cm round tart, a small tilt gives ~0.1cm difference; for a 60cm tart, the difference is ~1.05cm — comparable to the full depth. To avoid overflow at the low end, operators likely under-fill, further reducing filling volume.

Core Insight: Length Sells, Cross-Section Determines Satisfaction

Consumers judge size by the longest dimension (60cm looks impressive), but eating experience is determined by the cross-section (width, depth, pastry bulge). Optimization should focus on increasing width and depth or controlling pastry rise, not on length. Even a 40cm tart with a generous cross-section would feel more like a proper egg tart.

References: [1] Guangming Net, 2026-09-29; [2] Tencent News, 2026-09-28; [3] Galilei G., Dialogues Concerning Two New Sciences , 1638.