Stock and Ad Budget Calculator: Will You Run Out?

Use one product variant and one stock pool

Available stock must already exclude committed, unavailable and incoming units. Enter only an additional campaign reserve; if your inventory system already withholds it as unavailable, do not subtract it again. The horizon has no replenishment. Other demand is expressed in units per day and excludes the paid demand calculated separately. All money uses one currency.

Paid conversion is orders per paid click, not orders per session or all-store conversion. Units per order means units of this variant across those paid orders. For a single-variant campaign with one unit in every order it is 1. A broader campaign needs a defensible product allocation; guessing a site-wide average creates false precision.

The formulas behind the result

Let A be available units, R extra reserve, D days, O other units per day, S total spend, C cost per click, p paid conversion as a fraction and U units per paid order. Other demand is DO. The paid allowance is H=A−R−DO. Expected paid demand is Q=(S/C)pU. Ending stock is A−DO−Q; remaining above reserve is H−Q. A negative ending value represents unmet modelled demand, not physically negative stock.

For p greater than zero, the stock-based spend ceiling is max(0,H)C/(pU), rounded down to cents. If H is negative, other demand already breaches the allowance even with zero paid spend. With p=0 the ceiling is undefined, not an instruction to spend without limit. Days to reach the reserve is (A−R)/(O+Q/D) for a positive consumption rate. It assumes uniform consumption and is not a predicted date or probability.

Check the synthetic reference

A=500, R=60, D=14, O=10, S=12,000, C=1, p=0.02 and U=1 gives other demand 140, allowance 300, paid demand 240, ending stock 120 and ceiling 15,000. Uniform consumption reaches the extra reserve after approximately 16.21 days. At conversion 3%, paid demand is 360 and the ceiling falls to 10,000. The higher conversion assumption is a user-chosen stress case, not a confidence interval.

Validation, precision and limitations

Stock counts and days are integers; other inputs accept up to two decimals. Blank, negative, nonfinite, over-limit and malformed values are rejected. Reserve cannot exceed available. CPC and units per order must be positive. Changing any input clears stale results. Exact scaled-integer comparisons determine whether the allowance is exceeded; displayed results round to two decimals.

The JSON export contains inputs, anonymous scope and limitations. This local tool does not call an AI model or an inventory API. It excludes replenishment, returns becoming sellable, demand uncertainty, customer cancellations, spend-delivery variation and profit. Test a higher conversion, lower CPC and higher other-demand scenario before discussing a bounded budget with operations and finance.

Sources and evidence boundaries

Checked October 15, 2026: Shopify: understanding inventory states distinguishes available stock from committed, unavailable and incoming quantities. Google Merchant Center: availability requires consistent availability across product data and the customer journey, including the selected variant. Both are living documentation with an unknown publication date. The planning model and worksheet here are original editorial synthesis, not vendor-endorsed controls. All examples are synthetic; no measured search volume, stockout probability or performance improvement is claimed.