Inventory Carrying Cost Calculator for eCommerce

Key facts

• True cost of a purchase order = purchase cost + storage + shrinkage + cost of capital.
• Carrying costs typically run 8% to 18% of purchase value for sellers turning inventory 3 to 4 times a year.
• Amazon and Walmart storage fees roughly triple from October through December.
• The cash timeline runs from supplier deposit to the last payout, which can be four to six months.

Last updated September 7, 2026. Fee presets, payout timing, and Onramp Funds eligibility and fee example checked against the published sources listed at the end of this page.

How to use the inventory cost estimator

The supplier invoice is only part of what a purchase order costs. Open the supplier terms section to enter your deposit, lead time, and platform payout delay, and the calculator draws the full cash timeline for the order. This estimator adds storage, shrinkage, and the cost of the cash you tie up while units sit unsold, then gives you the true cost per sellable unit and the margin you actually keep. Enter one purchase order at a time.

• Cost per unit: landed cost, including freight, duties, and inbound handling.
• Storage per unit per month: warehouse rent divided by units on hand, your 3PL storage rate, or Amazon FBA monthly storage fees. Remember that FBA storage fees roughly triple from October through December.
• Months until sold through: how long it takes to sell the full order. The estimator assumes even sell-through, so storage is charged on the average units on hand.
• Shrinkage and damage: units lost to damage, expiry, theft, or unsellable returns. 1% to 3% is typical for durable goods, higher for perishables and fragile items.
• Cost of capital: an annual rate for what the cash could otherwise earn, or what it costs you to borrow it.

What makes this calculator different

Every number on this page has a time dimension. Other calculators tell you whether something is profitable; this one also shows the cash timeline: when the money goes out, when the platform pays you back, the week your cash position bottoms out, and how much you need to cover the gap. Three other things worth knowing:

• Platform presets: choose Amazon, Shopify, Walmart, TikTok Shop, WooCommerce, BigCommerce, Squarespace, Shopline, or Stripe and the fee and payout-delay fields fill in from published rate cards. Every preset is editable.
• Import your own export: upload a payments, settlement, or orders report from Amazon, Shopify, Walmart, or TikTok Shop and the calculator fills in revenue, fees, refunds, and payout timing from your actual data. The file is read in your browser and never uploaded anywhere.
• Compare two scenarios: duplicate your inputs into Scenario B, change one thing, and see both results and both cash curves side by side.
• Share or save: copy a link that reopens the calculator with your exact numbers, or save the result as a PDF. Your inputs carry over to the other calculators on this site, and nothing is stored on our servers.

The formulas

Purchase cost = units × cost per unit. Storage = average units on hand (half the order) × storage rate × months. Shrinkage = purchase cost × shrinkage percentage. Cost of capital = purchase cost × annual rate × months ÷ 12. Total holding cost = storage + shrinkage + cost of capital. True cost per sellable unit = (purchase cost + holding cost) ÷ sellable units.

What the holding cost percentage tells you

Holding costs below 8% of the purchase cost are lean. Between 8% and 18% is typical for a seller who turns inventory three to four times a year. Above 18% usually means either slow sell-through or high storage rates, and it is often cheaper to order smaller quantities more often even if the per-unit price goes up. The estimator shows what one month of faster sell-through saves on the current order.

Why inventory is the biggest cash problem in ecommerce

You pay the supplier before production, wait for freight, wait for the units to sell, then wait for the marketplace payout. That cycle can run four to six months, and it repeats every time you restock. Inventory is the single most common reason sellers come to Onramp Funds. Onramp advances capital against your sales across all nine supported platforms, with a flat fee disclosed upfront and repayment as a percentage of sales as the inventory sells. No personal credit check. See how much capital your combined revenue unlocks.

Sources

• Amazon selling fees and FBA rate card
• Walmart Marketplace referral and WFS fees
• Onramp Funds: how it works, eligibility, and fee example

Frequently asked questions

What is a good inventory turnover rate for ecommerce?

Four to six turns per year is healthy for most physical product brands. Below three turns, holding costs start to erode margin noticeably.

Should I include the cost of capital if I paid cash?

Yes. Cash sitting in inventory cannot fund ads, a second SKU, or a new channel. The rate you enter is the return that money could have earned elsewhere.

Does this estimator store my data?

No. Calculations run in your browser and nothing is saved.