Optimal product purchase volume for marketplaces: calculation and planning

Optimal purchase volume is the quantity of product sufficient to sustain sales until the next restock, without creating excess inventory in the warehouse. To calculate it, you need to factor in the average sales velocity, current remaining stock, lead times, seasonality and safety stock.

What the optimal purchase volume depends on

Buying products based only on last month's sales is not the most reliable strategy. Demand can change, the supplier may delay shipment, and some of the products may end up unsold.

When planning purchases, it's important to consider several metrics:

Sales velocity. How many units of a product are sold daily or weekly.

Current remaining stock. How many units are already in warehouses and available for sale.

Lead times. How long production, procurement and delivery of a new batch take.

Seasonality. How demand changes depending on the time of year, holidays and sales events.

Turnover. Over what period the inventory is sold through.

Safety stock. An additional quantity of product in case of supply delays or an unexpected surge in demand.

You also need to consider the supplier's minimum order quantity, storage costs and the available budget.

How to calculate the optimal purchase volume

For a basic calculation, you can use the formula:

Purchase volume = Average daily sales × (Lead time + Period between purchases) + Safety stock − Current remaining stock − Goods in transit

The formula accounts for goods in transit that will arrive within the calculation period. If the result is negative, a new purchase can be postponed for now.

Calculation example

Let's say a seller sells a product on Wildberries and Ozon. Their metrics are:

Metric	Value
Average sales	10 units/day
Lead time	12 days
Period between purchases	18 days
Safety stock	40 units
Current remaining stock	130 units
Goods in transit	50 units

First, we determine how much product will be needed for 30 days:

10 × (12 + 18) = 300 units

Add the safety stock and subtract the available remaining stock:

300 + 40 − 130 − 50 = 160 units

The recommended purchase volume is 160 units of product.

However, if the supplier ships products only in boxes of 50 units, you'll have to order 200 units or negotiate a change in the batch size.

How to tell when to purchase a new batch

It's important to calculate not only the quantity of product but also the moment to place the order. If you contact the supplier too late, your remaining stock may run out before the new batch arrives.

For this, the reorder point is used:

Reorder point = Average daily sales × Lead time + Safety stock

For example, with sales of 10 products per day, a lead time of 12 days and a safety stock of 40 units, the reorder point will be:

10 × 12 + 40 = 160 units

When the stock, including expected incoming shipments, drops to 160 units, a new purchase needs to be initiated.

How to avoid excess stock and product shortages

Even a correctly calculated purchase volume needs to be reviewed regularly. This is especially important for sellers with a large assortment, where some products sell daily while others can sit in the warehouse for months.

To optimize purchases, it's recommended to:

Analyze sales for each SKU. The overall store dynamics don't show which specific items need restocking.

Use ABC analysis. Group A products, which generate the bulk of gross revenue or profit, require more careful stock control.

Monitor turnover. If a product sells slower than expected, the next purchase should be reduced.

Account for seasonality. Before a high season, the purchase volume may need to be increased, and after the peak demand ends, reduced.

Evaluate the profitability of the purchase. Even a big discount from the supplier doesn't always justify buying a large batch if storage costs and tied-up capital exceed the savings.

At the same time, for new products with no sales history yet, it's better to use small test batches and adjust purchases as statistics accumulate.

How to calculate product purchases in Torgstat

When selling dozens or hundreds of products across multiple marketplaces, manual purchase calculations take a lot of time. Sales, remaining stock and supply data need to be constantly updated.

The Torgstat service has a "Supply calculation" tool that helps determine how much product is needed to replenish stock on Wildberries, Ozon and Yandex Market.

In the calculation, you can set:

the desired turnover period;

the period for which inventory needs to be provided;

sales velocity and its change coefficient;

production and logistics lead times;

the number of products per box;

the calculation basis — all orders or redeemed products.

Based on these parameters, the service calculates the recommended supply volume per product using sales and remaining stock data.

This allows you to plan warehouse replenishment, avoid shortages of popular items and avoid purchasing excess product.

FAQ

How do you determine the first purchase volume if there have been no sales yet?

For a new product, you can use category demand, sales of similar products and test results as a guide. It's better to keep the first batch small to test real demand and avoid tying up a significant sum in remaining stock.

For what period is it better to purchase product?

There's no universal timeframe. It depends on sales velocity, lead times, turnover and the supplier's terms. The faster you can replenish stock, the less need there is to create a large reserve.

How do you calculate a product's safety stock?

One simple way is to multiply average daily sales by the number of additional days that need to be covered. For example, with sales of 10 units per day and a 5-day reserve, the safety stock will be 50 units. For unstable demand, it's better to account for fluctuations in sales and lead times.

Why is it unprofitable to purchase overly large batches?

Large purchases increase the amount of funds invested in inventory. Storage costs may also grow, and if demand declines, there's a risk of having to sell the product at a discount.

Can the required quantity of product be calculated automatically?

Yes. For example, in Torgstat you can use the "Supply calculation" tool, which accounts for sales, remaining stock and the specified replenishment parameters. This is especially convenient when working with a large assortment across multiple marketplaces.