Forecasts are often carefully built, but still hard to use. You might know what you sold last year, what stock is arriving, or what ROAS you need. But when cash is tight and decisions are daily, you need more clarity.
This guide is for e-commerce founders who want a forecast that helps them act, not just observe. Here’s how to forecast Black Friday in a way that supports actual decisions with confidence.
1. Start with last year — but adjust for what’s changed
Last year’s numbers are useful, but only as a starting point. Pull sales, returns, contribution margin and ad spend from the previous Q4. Then update for what’s changed:
- New SKUs or categories
- Price changes or bundling strategies
- Changes to fulfilment, lead times or fees
- Updated refund or return behaviour
This gives you a baseline that reflects your current setup, not one that belongs to last year’s business.
2. Align your forecast to stock and supply
Your sales forecast means nothing if your stock plan can’t support it. Link your forecast to real inventory:
- Align forecasts to inbound shipments, lead times and purchasing constraints
- Include landed cost, not just supplier price
- Map bestsellers to run rate, ad spend and replenishment triggers
- Flag SKUs likely to overstock if they miss forecast
This gives you a realistic upper limit for sales, and shows what’s at risk of overstock if you miss target. It keeps the plan grounded in actual delivery capacity.
3. Link ad spend to margin and payback
Top-line revenue is not the target, contribution margin is. Marketing spend should be tied to margin and cash. Build your forecast around:
- Known ROAS and payback periods by channel
- Contribution margin after ad spend, not just gross margin
- Cash burn rate and headroom during peak weeks
- The breakeven point where spend stops scaling profitably
With this in place, you can set a hard ad budget or scale it dynamically with confidence. It lets you set guardrails: how much you can push spend before returns flatten or cash runs too low.
4. Layer in cash timing and VAT
Sales peaks often create cash gaps. On 29 November you’ll feel cash-rich. On 7 February you might feel sick. That’s VAT plus January refunds hitting at once. Model that now so you’re not surprised later.
Black Friday volume brings VAT, fulfilment and refund spikes. Include:
- Payout timings from Stripe, PayPal, Amazon, especially over the Black Friday period
- VAT due on Q4 sales and when it hits the bank
- January return spikes and their cash impact
- Supplier payments that land during peak trading
- Import duty and VAT on inbound stock, especially if you’re paying the import VAT up front rather than using postponed accounting
5. Turn it into a weekly decision tool
A forecast isn’t useful unless you can act on it in real time. Build:
- Weekly cashflow view through January
- Stock position with triggers to reorder or hold
- Ad spend caps tied to contribution margin
- KPI dashboards to track margin, ROAS, stock turn
This is what makes the forecast usable. It’s a tool for operators.
Also see our complete guide on: Financial Planning and Forecasting for E-Commerce Brands
Want help building one?
Elver E-Commerce Accountants works with online retail founders to build forecasts that link stock, marketing, VAT and cash into one plan. If you need a scenario that actually helps you decide what to spend and when to reorder, we can help.
Book a short consultation and we’ll walk you through the forecasting process.