How to Stop Running Out of Your Best-Selling Products (Without Overbuying)

If you keep finding yourself struggling to stay on top of your ordering or constantly running out of your best sellers, this blog is for you.

I’m Katie, a Stock & Profit Specialist with almost 20 years of retail experience, working with brands like Mulberry, Warehouse, and Clarks, as well as running my own small business. These days, I help other small business owners make sense of their numbers and get their stock working harder for them.

Running out of your best sellers can cause chaos for your cash flow. They’re usually the products that keep your business ticking over, so keeping them in stock is key. If managing your inventory feels like a bit of a guessing game, you’re in the right place.

Here are some simple, practical steps to help you stay on top of your ordering, keep your best sellers available, and protect your profits.

Identify Your Best-Selling Products

Before you can stop running out of stock, you need to know which products are worth keeping an eye on and topping up regularly.

Start by identifying your true best sellers, the products that drive most of your sales or profit and so you can't be out of stock of. In most small businesses, around 20% of your products will make up roughly 80% of your revenue, so keeping these well-stocked is the quickest way to protect your sales (and your sanity).

You can call these your “never out of stock” or “A-grade” products. Whatever name you choose, these are the ones that deserve your time, attention, and money. If you only have an hour to place orders this week, check these products first.

Don’t forget to include your key add-ons in this group, products that might not be your top sellers on their own but are essential to your brand experience or are often bought alongside others. These could include:

• Care products like suede protector for shoes or wick trimmers for candles
• Picture frames or mounts for art prints
• Batteries for electronic products

These items might not generate the highest revenue individually, but they boost your average order value and strengthen your brand perception.

Once you’ve identified your top products, categorise the rest of your range:

• B-grade products – Steady sellers that contribute to your sales but don’t need constant reordering.
• C-grade products – Slow movers that tie up cash. Keep an eye on these and consider phasing them out.

💡 Id you have a Shopify store, try looking at Shopify’s ABC Inventory Report that grades your products for you:

• A products = ~80% of your revenue (your top sellers)
• B products = ~15% of your revenue (steady sellers)
• C products = ~5% of your revenue (slow movers)

Just make sure to sense-check what the report’s telling you. Something that’s been on sale might look like a top seller, or a product that’s been out of stock could seem like it’s not performing well. Treat it as a guide, not gospel, your own insight always matters most.

Forecast Your Sales to Take the Guesswork Out of When and How Much to Buy

Once you know which products to focus on, the next step is to work out when to reorder and how much to buy.

If you’ve been trading for a while, your sales history is your best friend. Use it to forecast demand by looking at last year’s sales and applying an uplift based on your current growth.

Example:

• Average weekly sales = 10 units
• Year-to-date sales growth = 20% (1.2 uplift)
• 10 × 1.2 = 12 units forecasted per week

If you don’t have last year’s data, that’s fine. Use your recent weekly or monthly averages instead. Just remember to adjust for seasonal peaks and key events like Black Friday, Christmas, or Mother’s Day.

💡 To learn more about forecasting your sales, check out my other blog, 3 Ways to Forecast Sales for Q4 (and Other Busy Seasons in Your Small Business).

Step 1. Work Out Your Reorder Point

Once you’ve forecasted your sales, you can use that data to work out when to reorder before you run out.

A reorder point is the stock level that triggers your next purchase order.

Formula:
Reorder Point = Average Weekly Sales × Supplier Lead Time (in weeks) + Safety Stock

Example: If you sell 12 units per week, your supplier takes 1 week to deliver, and you want to add half a week’s buffer, your reorder point would be 18.

• 12 (weekly sales) × 1 week lead time = 12
• 6 units of safety stock (half a week’s sales)
  = 18 units is your reorder point

Step 2. Calculate How Much to Order

Once you know when to reorder, you need to decide how much to buy.

Formula:
Order Quantity = Forecasted Sales (for your lead time and stock cover period) – Current Stock and Committed Stock

Example: Now let’s say you want to hold a month’s worth of stock, and you already have 18 units in stock:

• 12 units × 5 weeks (4 weeks of cover + 1 week lead time) = 60 units
  60 (forecast) – 18 (current stock) = 42 units to order

Step 3. Adjust for Stock Gaps

If you know you’ll sell out while waiting for new stock though, don’t include those “out of stock” weeks in your forecast.

Example: If you currently have 0 stock and your supplier takes 1 week to deliver:

12 units × 4 weeks (excluding the 1 week you’ll be out of stock) = 48
48 (forecast) – 0 (current stock) = 48 units to order

Including those lost sales weeks in your forecast would make you overstocked when it arrives.

Choose the Right Method for Your Product

This method works best if your sales are steady and you’re not expecting a big peak.

If your sales are more seasonal or fluctuate a lot, you might find it easier to create a weekly or monthly buying plan in a spreadsheet. This lets you see out-of-stock weeks visually and tweak your forecasts as you go, perfect if you prefer a more hands-on approach, but I'll be talking in more detail on those later!

💡 Tip: Review your reorder points and forecasts regularly, especially during busy seasons or if your supplier lead times change. A quick adjustment now can save you from last-minute stock panics later.

Set Up Flows in Shopify to Get Automatic Low Stock Alerts

Once you’ve worked out your reorder points, you can take it one step further and automate your low stock alerts in Shopify. Using the free Shopify Flow app, you can create a simple automation that sends you an email when a product’s stock drops below a certain level, so you know exactly when to reorder.

No fancy paid apps or subscriptions needed!

You’ll just need to store your reorder point in Shopify using metafields, then build a quick Flow that emails you when stock hits that level.

It takes a little time to set up the first time, but once it’s running, you’ll never have to rely on guesswork or last-minute supplier calls again. You’ll have peace of mind knowing your best sellers are always topped up and your cash flow is protected.

For a full step-by-step guide, including examples and screenshots, check out my other blog, How to Set Up Low Stock Alerts in Shopify (Without Paying for an App): Part 1.

Create Product Buy Plans or Line Cards For your Best Sellers

This does involve a little bit of spreadsheet magic, but don’t worry, minimal formulas required, I promise!

Creating a product buy plan (sometimes called a line card) is a great way to plan your stock by month and take seasonal peaks into account. It helps you see exactly when you’ll need to reorder, where you might run out, and how your stock levels will look over time.

Start by forecasting your monthly sales in one column, then plug in your current stock and any future deliveries you already have planned in another column.

Use your spreadsheet to project what this means for your monthly closing stock levels  in your final column so you can easily spot any gaps that need filling.

Formula:
Month closing stock = Month opening stock – Sales + Stock deliveries that arrived that month

If your month-end stock goes negative or looks too low, that’s your signal to order extra stock for that month.

Or, if you know you can’t get any more stock in that month (which means you’ll go out of stock), adjust your sales forecast for that month until your closing stock is zero. That way, you’re not overforecasting or overbuying based on sales you can’t actually fulfil.

If the thought of setting up a spreadsheet makes your head spin, don’t worry, I offer a service where I can build a personalised buy plan for your products, sales patterns, and supplier lead times.

Hold Extra Safety Stock to Protect Against Supplier Delays

I’m not saying you need to buy double what you usually do, but it’s worth keeping a little safety stock of your best sellers, perhaps an extra week or two of sales, to protect yourself from sudden spikes in demand or unexpected supplier delays.

This is where categorising your products into A, B, and C grades really helps. You can adjust your buffer depending on how important or fast-selling each product is. For example:

• A-grade products: Your best sellers that should always be in stock. Keep a higher buffer for these.
• B-grade products: Steady sellers where a smaller buffer might be enough.
• C-grade products: Slow movers that don’t need any buffer at all.

How much safety stock you need will depend on how reliable your suppliers are and how long your lead times are.

For example:

• If your supplier delivers within a week, you probably only need a few days’ extra stock.
• If your lead time is a month, you might want to hold two weeks’ worth of extra stock.

There’s no hard and fast rule. The best approach is to base your buffer on real experience, the delays you’ve had, the sales spikes you’ve seen, and what’s realistic for your business.

As your business grows, keep adjusting this. If your suppliers are consistent and demand stays steady, you can reduce your buffer. If your sales are unpredictable or your suppliers tend to run late, you might need to increase it to 75% or even 100% of your lead time to stay covered.

What to Do When You Still Run Out of Stock

Even with the best planning, sometimes shiz still happens, and you run out of your best sellers. It’s part of business life!

Hopefully, some customers will be happy to wait for a restock, but if it’s a time-sensitive item, like a gift, you’ll want to make sure they don’t click away to another shop.

To minimise the impact of going out of stock and protect your sales, here are a few simple things you can do:

• Let customers know when stock is due
  Add a message on your product page such as “Back in stock on [date]” so customers know when to expect it. Transparency helps build trust and keeps people coming back.
• Add a waitlist or back-in-stock alert
  Use Shopify’s built-in email capture or an app that lets customers sign up to be notified when an item is available again. This way, you don’t lose potential sales — you just delay them.
• Suggest similar alternatives
  Add links to related products or recommend similar styles. If someone can’t get the exact item they wanted, they might still buy something else they love.

Running out of stock isn’t the end of the world. What matters most is how you handle it. Communicate clearly, stay proactive, and use it as a chance to strengthen customer trust rather than lose it.

Stay in Control of Your Stock and Cash Flow

Managing your stock doesn’t have to feel like a guessing game. With a few simple systems in place, you can take back control of your inventory and protect your profits.

Start by identifying your best sellers and keeping them well stocked, then use your sales data to forecast what you’ll need and when. Add a little safety stock for peace of mind, and create a simple buy plan or line card to see where gaps might appear throughout the year.

Once you’ve got the basics in place, take things a step further by automating your low stock alerts in Shopify. It’s a free and easy way to stay ahead, without relying on memory or last-minute supplier calls.

And if things don’t go to plan and you do run out, communicate clearly with your customers. Offer alternatives, waitlists, or restock notifications to keep them engaged and returning.

Small steps like these can make a big difference to your cash flow, confidence, and sanity. The goal isn’t to be perfect,  it’s to stay organised, make better decisions, and give yourself more breathing space in your business.

If you’d love a hand setting up your Shopify stock systems or building a buy plan that makes sense for your products, get in touch. I’d love to help you make stock management simple, calm, and profitable.