What is Size Curve Planning?

Definition

Size Curve Planning is the process of forecasting how many units of each size a business should purchase, produce, or allocate to match expected customer demand. It is especially important for apparel, footwear, and other size-dependent merchandise where total unit demand does not explain which sizes will sell.

The plan converts historical sales, current inventory, seasonal patterns, regional demand, and assortment strategy into a practical size distribution. Finance and merchandising teams can use the resulting curve to align inventory investment with expected demand while reducing excess stock in slower-moving sizes.

Table of Contents

How Size Curve Planning Works

Size Curve Planning starts with historical sales or demand data by size. Teams examine the proportion of total demand represented by each size and adjust the pattern for changes in product mix, geography, seasonality, promotions, and customer behavior.

  • Demand history: Measures the historical share of sales generated by each size.
  • Current inventory: Shows which sizes are already available and where replenishment is needed.
  • Seasonal adjustments: Account for changes in size demand across selling periods.
  • Assortment targets: Translate the expected size mix into purchase and production quantities.

The resulting curve can be applied at product, category, store, region, channel, or season level. A detailed plan may use different curves for different markets rather than applying one company-wide ratio.

Size Curve Calculation and Example

A basic size curve calculates each size's percentage of total units sold:

Size Demand Percentage = Units Sold for a Size ÷ Total Units Sold × 100

For example, assume a retailer sold 1,000 jacket units consisting of 100 small, 300 medium, 350 large, and 250 XL units. The large-size share is 350 ÷ 1,000 × 100 = 35%.

If the retailer plans to purchase 2,000 jackets for the next season and initially applies the same curve, approximately 700 units would be allocated to large sizes. The final purchase plan can then incorporate safety stock, regional differences, and expected changes in demand.

Size Curve Planning and Inventory Allocation

Size curves help merchandising and inventory teams translate total assortment budgets into specific unit quantities. Instead of purchasing an equal number of every size, planners can prioritize quantities according to expected demand.

This approach also supports allocation after inventory arrives. If medium and large sizes historically account for most demand, distribution centers can prioritize those sizes for stores or channels with stronger demand. Actual sell-through should then be compared with the planned curve so future allocations can be adjusted.

Size Curves, Procurement, and ERP Integration

Once a size plan is approved, it can inform requisitions, purchase order quantities, supplier commitments, and inventory budgets. Procurement teams can use the curve alongside sourcing decisions to determine how much merchandise should be acquired and from which suppliers.

ERP integration can connect size-level planning with purchasing, inventory, sales, and financial records. Businesses using oracle or another ERP can extend finance workflows so planned quantities remain connected to inventory and purchasing data. For online retailers, eCommerce ERP Software: Complete 2025 Guide to ERP Webshop can provide relevant context for connecting commerce operations with ERP processes.

Size Curve Planning and Financial Decisions

Size-level planning affects more than merchandise availability. It influences inventory investment, working capital, markdown exposure, purchasing commitments, and expected gross margin. A curve that closely reflects demand can help allocate available inventory funds toward sizes with stronger expected turnover.

Finance teams can also monitor the relationship between planned and actual size demand. Large deviations may indicate that forecasts, regional assumptions, product attributes, or purchasing quantities need adjustment. These comparisons strengthen inventory budgeting and improve the quality of future financial decisions.

Operational finance workflows can also benefit from AP Automation Software when size-driven purchasing generates substantial invoice volume, because invoice processing and payment planning remain connected to controlled accounts payable operations.

Size Curve Planning concerns merchandise demand rather than interest rates, but finance teams may encounter similarly named curve terminology. A Yield Curve represents interest rates across different maturities, while Yield Curve Analysis examines the shape and movement of those rates for financial interpretation.

Yield Curve Risk concerns the potential financial impact of changes in the relationship between interest rates at different maturities. Keeping these concepts distinct prevents confusion when size-planning terminology appears alongside broader financial analysis.

Best Practices for Size Curve Planning

  • Build curves from sufficiently detailed historical sales data rather than relying only on total category demand.
  • Review curves separately for major products, regions, channels, and seasons when customer behavior differs.
  • Compare planned size distributions with actual sales and inventory balances after each selling period.
  • Coordinate size curves with purchasing approvals, inventory budgets, supplier lead times, and replenishment policies.
  • Refresh curves when product design, pricing, customer demographics, or market conditions materially change.

Summary

Size Curve Planning converts expected demand by size into actionable purchasing, production, and allocation quantities. By combining historical demand with inventory, seasonal, regional, and financial considerations, businesses can improve assortment decisions, inventory utilization, and working-capital planning.