One of the commonly asked questions by inventory managers is: "How much inventory should I order?"

The right order quantity can be calculated using a combination of demand forecasting, lead times, safety stock, and inventory costs.

This guide explains how to determine the right amount to order, and when to use each calculation.

Key Takeaways

• There is no universal "right" inventory quantity.
• Use the Economic Order Quantity (EOQ) formula to minimize ordering and holding costs.
• Calculate a Reorder Point to know exactly when to place an order.
• Maintain Safety Stock to protect against demand fluctuations and supplier delays.
• Review your inventory calculations regularly as demand and supply conditions change.

How Much Inventory Should I Order?

There isn't a universal "correct" inventory quantity for every business.

That’s because your ideal order quantity typically depends on the following:

• Forecasted demand
• Supplier lead time
• Current inventory
• Safety stock requirements
• Ordering costs
• Inventory holding costs
• Supplier minimum order quantities (MOQs)

For businesses with a relatively stable demand, the Economic Order Quantity (EOQ) formula is often the best starting point because it minimizes the combined cost of ordering and carrying inventory.

Step 1: Forecast Your Demand

Before deciding how much to order, estimate how much you'll sell before your next replenishment.

For example:

• Average monthly sales = 600 units

• Supplier lead time = 30 days

You'll likely sell approximately 600 units while waiting for your next shipment.

If you’re dealing with products with seasonal demand, use forecasted demand instead of historical averages as customer demand constantly changes and using historical averages might be highly inaccurate.

Step 2: Calculate Your Economic Order Quantity (EOQ)

EOQ is one of the oldest and most widely used inventory planning formulas.

It answers the question: How much should I order each time?

The formula is:

EOQ = √((2 × Annual Demand × Ordering Cost) ÷ Holding Cost)

Where:

• • Annual Demand (D) = units sold each year

  • Ordering Cost (S) = cost of placing one purchase order

Holding Cost (H) = annual cost of storing one unit

Example of an EOQ Calculation

Suppose you sell:

• Annual demand = 12,000 units

• Cost to place an order = $50

• Holding cost = $2 per unit per year

EOQ:

√((2 × 12,000 × 50) ÷ 2)

= √600,000

≈ 775 units

This means ordering around 775 units at a time minimizes the total cost of ordering and holding inventory.

Step 3: Don't Forget Your Reorder Point

Knowing how much to order is only half the equation.

You also need to know when to place the order.

The standard reorder point formula is:

Reorder Point = (Average Daily Demand × Lead Time) + Safety Stock

Example:

• Average daily sales = 20 units

• Lead time = 10 days

• Safety stock = 50 units

Reorder Point: (20 × 10) + 50 = 250 units

Once inventory reaches 250 units, it's time to place your next purchase order.

Step 4: Calculate Safety Stock

Demand is rarely perfectly predictable.

Suppliers may also deliver later than expected.

Safety stock acts as a buffer against both.

A commonly used statistical formula is:

Safety Stock = Z × σ × √Lead Time

Where:

• Z = desired service level

• σ = standard deviation of demand

• Lead Time = supplier lead time

Typical service levels are:

Higher service levels reduce stock-outs but require carrying more inventory.

When Does EOQ Not Work Well?

EOQ assumes:

• Stable demand

• Consistent lead times

• Constant ordering costs

• Constant holding costs

However, many businesses experience:

• Seasonal demand

• Promotional spikes

• Supplier delays

• New product launches

• Supply chain disruptions

In these situations, dynamic demand forecasting generally produces better purchasing decisions than relying on a fixed EOQ alone. Modern inventory planning systems continuously update recommendations as sales patterns change.

Other Factors That Affect Order Quantity

Even if your calculations suggest ordering 775 units, real-world constraints may require adjustments.

Here are some factors to take into consideration:

Supplier Minimum Order Quantities (MOQ)

Some suppliers require purchases in fixed case packs or minimum quantities.

Cash Flow

Ordering more inventory ties up working capital that could be invested elsewhere.

Warehouse Capacity

Ordering more than you can store increases storage costs and handling complexity.

Product Shelf Life

Perishable or trend-driven products should generally be ordered more frequently in smaller quantities.

Bulk Discounts

Large discounts aren't always cheaper once carrying costs are considered. Compare the total cost - not just the purchase price.

A Simple Rule of Thumb

If you're not yet ready for advanced inventory optimization, here’s a framework to consider:

• Forecast expected sales.

• Add an appropriate safety stock buffer.

• Calculate your reorder point.

• Use EOQ as your initial order quantity.

• Review your forecasts every month.

This approach is significantly more reliable than ordering based on intuition or simply repeating your last purchase order.

Example

A retailer sells approximately 600 units per month.

• Current inventory: 900 units

• Supplier lead time: 30 days

• Safety stock: 200 units

Demand during lead time: 600 units

Reorder Point: 600 + 200 = 800 units

When inventory falls to 800 units, it’s time to place a new order.

Using EOQ calculations, the recommended purchase quantity is 775 units.

This balances inventory availability while minimizing overall inventory costs.

Common Inventory Planning Mistakes Businesses Make

Avoid these common inventory planning errors:

• Ordering based on instinct rather than data
• Ignoring supplier lead times
• Using outdated sales history
• Holding the same safety stock for every SKU
• Never reviewing reorder points
• Buying in bulk simply because of supplier discounts

Small improvements in forecasting accuracy often translate into lower inventory costs and higher product availability.

Conclusion

By combining demand forecasting, EOQ, reorder points, and safety stock calculations, businesses can make smarter purchasing decisions while maintaining high service levels.

However, as your product range grows, demand becomes more complex and doing this manually will melt your brain.

StockTrim is a lightweight inventory forecasting platform built specifically for SMBs. Since 2017, we've analysed tens of thousands of real SME inventory datasets to help businesses generate more accurate demand forecasts and purchasing recommendations.

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Frequently Asked Questions

1) Is there a formula for how much inventory to order?

Yes. The most widely used method is the Economic Order Quantity (EOQ) formula: EOQ = √((2 × Annual Demand × Ordering Cost) ÷ Holding Cost)

It identifies the order quantity that minimizes total inventory costs.

2) How often should I recalculate order quantities?

Review your inventory planning whenever:

• demand changes significantly,

• supplier lead times change,

• inventory costs increase,

• new products are introduced, or

• at least once every month for fast-moving products.

3) Should every SKU have the same order quantity?

No. High-volume products, seasonal items, slow-moving inventory, and expensive products should each have different replenishment strategies.