The easiest, most traditional calculation

Many small businesses simply use a percentage of revenue as their guide for how much to spend on advertising. If you're going to use this method, you need to figure out two things: what percentage, and what revenue? Rather than generalizing (5% of your revenue) it's better to use your industry's average percentage as a guide. For the most accurate number, base the percentage on an average of the last few years' revenues, or on the average of last year's revenues and your projection of next year's revenues.

I recommend: To find your industry’s average percentage of revenue spent on advertising, search Dun & Bradstreet  or Canada's  Strategis database. Still not finding it? Some trade magazines track detailed financials. To find your trade magazine, search TradePub.com.

The unit-of-sales method

This method relies on your experience, plus averages for your industry, of how much it costs to sell a certain specific product. Determine the number of such products you want to sell, multiply that number (1000 widgets X five cents per widget) and you'll get your total budget ($50). The unit of sales method works well for companies with only a few different products to sell, and for product lines with limited or circumscribed availability (an artist can make X number of carvings per year with one apprentice, for example).

I recommend: Your need in this instance is to get good key numbers – try your industry trade association. Search for it on ASAE and The Center for Association Leadership’s online directory or on the U.S. government’s Consumer Action Website.

Tie advertising investments to rent expenses

Roy Williams, an advertising author and columnist for Entrepreneur magazine, suggests an unusual way to calculate ad budgets, which should work well for retailers and others whose facilities play a role (any role, limited or significant) in their brand identity. The calculation, which is slightly too complex to repeat here, will provide you with a very specific budget range.

I recommend: Follow Williams’ tutorial, “A New Way to Calculate Your Ad Budget,” at Entrepreneur.com.

Calculate your budget by task and objective

Although this method is the hardest one to use, most experts agree that it's the most accurate way to come up with the right ad budget. It's hard because you must first create a marketing plan. What are your objectives? Rank them in order of importance, because you may find that you can't afford to achieve all of them. Determine: 1) who you want to reach, 2) how frequently you want to reach them, 3) where (through which combination of media) you'll get the best exposure, and 4) how much it will cost. Add it all up. Too much money? Reduce your objectives.

I recommend: To use the task and objective method, you need a good marketing plan. Find numerous sample marketing plans at MPlan.com. Or, if you need more structure, get it from Marketing Plan Pro software. Want professional help? Search for an advertising agency on agencyfinder.com or salesvantage.com.

Online tools make it easier

There are a variety of calculators online, geared to different industries. Just plug in your numbers and go!

I recommend: Want to figure out how much your advertising impressions will cost? Marketing Today’s Advertising CPM (cost-per-thousand) calculator is useful. Search engine marketers can figure out keyword ROI on the Web Marketing ROI calculator.

Search engine marketing or pay-per-click

We'd be remiss not to mention pay-per-click in a guide on calculating your ad budget. Nothing has made the analysts who work in the advertising industry happier than the rise of pay-per-click ads and other search engine marketing techniques, because it's easy to measure their return on investment - the Holy Grail of advertising.

I recommend: For a great tutorial on measuring your pay-per-click ROI, or "return on ad spend" (ROAS) see Google Adwords Learning Center.