What Is a Reorder Point? How to Calculate It for Your Store

A reorder point (ROP) is the inventory level at which you should place a new order with your supplier. When your stock drops to this number, it's time to reorder — not before, not after. The goal is to replenish stock before you run out, while accounting for how long it takes your supplier to deliver.

That's the core of it. But the difference between a reorder point set correctly and one pulled from thin air can mean the difference between a smooth Q4 and a stockout that costs you thousands in lost sales.

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Why Reorder Points Matter for Ecommerce Stores

Most Shopify merchants start with a gut feeling: "When I have 20 units left, I'll reorder." That works when you have five SKUs and a two-day supplier. It breaks down fast when you're managing 200 SKUs with lead times ranging from 7 to 45 days — or when your sell rate doubles during a promotion.

The reorder point formula exists to replace the gut feeling with math. It takes three variables into account:

1. Average daily sales — how many units you typically sell per day
2. Lead time — how many days it takes to receive a new order from your supplier
3. Safety stock — a buffer to protect against demand spikes or supplier delays

Get all three right, and you'll almost never stock out. Get one wrong, and you'll either run out mid-lead-time or tie up cash in excess inventory you don't need.

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The Reorder Point Formula

The standard formula is:

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

Let's work through a concrete example.

Say you sell a moisturizer that moves 15 units per day. Your supplier takes 10 days to fulfill. You want to keep 3 days' worth of safety stock as a buffer.

Reorder Point = (15 × 10) + (15 × 3)
Reorder Point = 150 + 45
Reorder Point = 195 units

When your inventory hits 195 units, you place the order. By the time it arrives 10 days later, you'll have burned through roughly 150 units — leaving you with 45 units of safety stock still on hand as a cushion.

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How to Calculate Safety Stock

Safety stock is often treated as a fixed number pulled from intuition ("let's keep 50 units extra"), but there's a better way to calculate it based on variability in your demand and lead time.

A simple formula:

Safety Stock = Z × σ × √Lead Time

Where:

• Z = your desired service level (1.65 for 95%, 2.05 for 98%)
• σ = standard deviation of daily sales
• Lead Time = days to receive stock

This gets statistically precise, but for most DTC brands it's overkill. A practical shortcut that works well:

Safety Stock = (Max Daily Sales − Average Daily Sales) × Lead Time

If your moisturizer peaks at 25 units/day during promotions but averages 15, and lead time is 10 days:

Safety Stock = (25 − 15) × 10 = 100 units

That's a more conservative buffer — appropriate for a fast-moving SKU where a stockout is expensive. For slow-moving products with more predictable demand, you can trim this significantly.

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Reorder Point With Multiple Suppliers or Variable Lead Times

One reorder point formula assumes a consistent lead time. Real-world suppliers often don't deliver consistently — your overseas manufacturer might say "3-4 weeks" and mean anywhere from 18 to 35 days.

When lead times vary, use average lead time in your formula but build it into your safety stock calculation instead:

Safety Stock = Z × Average Daily Sales × Standard Deviation of Lead Time

For a store ordering from a supplier with an average 14-day lead time but a standard deviation of 4 days, and selling 20 units/day at 95% service level:

Safety Stock = 1.65 × 20 × 4 = 132 units
ROP = (20 × 14) + 132 = 412 units

That's a meaningful buffer — and it reflects the actual risk of that supplier relationship. If you switched to a more reliable supplier with a 2-day standard deviation, your safety stock would drop to 66 units, freeing up real cash.

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Common Mistakes When Setting Reorder Points

Using a static average that's months old. If you calculated your daily sales average in January and it's now November, that number is probably wrong for most SKUs. Seasonality matters. A candle that moves 5 units/day in summer might move 40/day in December.

Not accounting for promotional spikes. If you run a 20% off sale every Black Friday, your standard daily sales average will underpredict demand. Build a higher-than-usual safety stock before known promotions.

Setting the same lead time for all suppliers. Some stores use a blanket "14-day lead time" across the board. But if one supplier ships from a domestic warehouse in 3 days and another ships from overseas in 21, you're either holding too much stock for the fast supplier or cutting it too close with the slow one.

Ignoring minimum order quantities (MOQs). Your ROP calculation tells you when to order. Your supplier's MOQ dictates how much you'll receive. If your ROP says order when you hit 200 units and your supplier has a 500-unit MOQ, you need enough storage to hold the incoming order before your current stock runs out.

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How to Track Reorder Points in Practice

Knowing the formula is step one. Actually monitoring inventory levels against your reorder points in real time is where most stores fall short.

There are a few ways to do it:

Manual spreadsheet tracking. Workable for fewer than 20 SKUs. You export your Shopify inventory weekly, compare current levels to your calculated ROPs, and flag anything that needs ordering. It breaks down quickly as SKU count grows because the data is always slightly stale.

Shopify's built-in low stock reports. Shopify does let you set a low stock threshold per product, but it doesn't calculate your reorder point based on sales velocity or lead time. It's a static threshold — you set 20 units, it alerts you at 20 units, regardless of whether 20 units represents two days of stock or two months.

Dedicated inventory apps. Tools like Alertr, Bee Low Stock Alert, or Prediko connect to your Shopify store and monitor inventory levels dynamically. Alertr, for instance, calculates your sell rate and days of stock remaining per SKU and sends alerts via email or Slack when you're approaching a reorder point — so instead of checking a spreadsheet, you get a notification that says "SKU-1047 has 8 days of stock left based on current sell rate."

For a store managing 100-500 SKUs, that kind of automation saves real time. Prediko takes it further with full purchase order management but starts at $49/month. Bee Low Stock Alert offers a simpler, cheaper option at $5.99/month. Alertr sits in between — built specifically for DTC Shopify brands that need accurate sell-rate tracking and configurable alerts without the complexity of a full inventory planning suite.

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Reorder Point vs. Reorder Quantity: A Key Distinction

These are often confused. The reorder point answers when to order. The reorder quantity answers how much to order.

The Economic Order Quantity (EOQ) formula is commonly used to determine optimal order quantity:

EOQ = √(2DS / H)

Where D = annual demand, S = ordering cost per order, and H = holding cost per unit per year. This balances the cost of ordering frequently (labor, shipping) against the cost of holding inventory (storage, tied-up capital).

For most small-to-mid DTC brands, supplier MOQs make EOQ less relevant in practice — you order the minimum you can afford, not the mathematically optimal amount. But as you scale past 1,000 orders/month, optimizing order quantities can meaningfully reduce your working capital requirements.

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Putting It Together: A Simple ROP Workflow

Here's a practical process you can implement this week:

1. Pull your last 90 days of sales by SKU from Shopify. Calculate average daily sales and the standard deviation (Excel's STDEV function works fine).

2. List your lead times by supplier. If you don't know the exact number, look at your last 5 purchase orders and calculate the average days from order to receipt.

3. Calculate safety stock using the simpler formula: (Max Daily Sales − Average Daily Sales) × Lead Time.

4. Calculate your reorder point per SKU: (Average Daily Sales × Lead Time) + Safety Stock.

5. Set alerts so you know when any SKU crosses its reorder point. Whether that's a Shopify threshold, a spreadsheet flag, or an app like Alertr sending you a Slack message, the mechanism matters less than the consistency.

6. Revisit quarterly — or before major sales events. Your sell rates and supplier lead times change, and your reorder points need to keep up.

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Final Thoughts

A reorder point isn't a set-and-forget number. It's a dynamic threshold that reflects your current sell rate, your supplier's current reliability, and the cost of getting it wrong. The formula is simple, but the discipline of keeping it current is where most stores struggle.

If you're managing more than 50 SKUs, that discipline gets hard to maintain manually. Alertr was built for exactly this — it tracks your sell rate per SKU automatically, estimates days of stock remaining, and alerts you via email or Slack when it's time to reorder. There's a free tier for up to 50 SKUs, and the Pro plan is currently $19/month (locked forever for early users).

Whether you use a tool or a spreadsheet, the important thing is to get off gut feeling and onto math. Your inventory — and your cash flow — will thank you.

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