Ad Budget Calculator for Ecommerce — Plan Your Monthly Ad Spend

Setting an ad budget is one of the most stressful decisions for ecommerce founders. Spend too little and you will not gather enough data to optimise. Spend too much without a plan and you will burn through cash before the algorithms have time to learn. The calculator above takes the guesswork out by working backwards from your revenue goals.

The core equation is: Required Ad Spend = Revenue Target / Expected ROAS. If you want to generate £20,000 in monthly revenue and expect a 3x ROAS, you need to budget approximately £6,667 per month, or about £222 per day.

Why Starting Budget Matters More Than You Think

Advertising platforms like Meta and Google use machine learning to optimise your campaigns. These algorithms need data to learn who your best customers are and what creative resonates with them. If your daily budget is too low, you will not generate enough conversions for the algorithm to exit the learning phase.

Meta recommends generating at least 50 conversions per week per ad set for optimal performance. If your CPA is £15, that means you need at least £750 per week (approximately £107 per day) per ad set. Running five ad sets at £20 per day each will likely perform worse than one ad set at £100 per day.

The Conversion Rate Factor

Your website conversion rate has a massive impact on how much you need to spend. A store converting at 1% needs twice as many clicks (and therefore roughly twice the budget) as a store converting at 2% to generate the same number of orders.

Before scaling ad spend, make sure your conversion rate is solid. Industry benchmarks for ecommerce are:

• Below 1%: There are likely fundamental issues with your site, offer, or pricing. Fix these before spending heavily on ads.
• 1-2%: Average for most ecommerce stores. Viable for advertising but room for improvement.
• 2-4%: Good performance. Your store is ready to scale with paid traffic.
• 4%+: Excellent. You are in a strong position to scale aggressively and absorb higher CPCs.

How to Phase Your Budget

If you are new to paid advertising, do not dump your entire monthly budget in on day one. A phased approach reduces risk:

• Week 1-2 (Testing): Start at 30-50% of your planned daily budget. Run multiple ad creatives and audiences. Focus on gathering data, not hitting ROAS targets.
• Week 3-4 (Optimising): Cut underperforming ads and audiences. Scale budget to 70-100% on what is working. You should start seeing more consistent results.
• Month 2+ (Scaling): Increase budget by 20-30% per week on winning campaigns. Introduce new creative and audiences to prevent fatigue.

Budget Allocation Across Platforms

If you are running ads on multiple platforms, a common starting split for ecommerce brands is 60-70% on Meta (Facebook and Instagram), 20-30% on Google (Shopping and Search), and 10% for testing new channels like TikTok or Pinterest. Adjust based on where your customers spend their time and which platform delivers the best ROAS for your specific products.

The key is to track performance by platform and shift budget towards what works. Do not spread budget evenly across five platforms just for the sake of diversification. Concentrate spend where it generates returns, then expand.