Email marketing is an affordable marketing tool for converting prospects into buyers and buyers into repeat buyers but, unless done well, your email marketing efforts will fall flat. Here are four essential email marketing metrics and four hacks for succeeding at each.

1. Email open rates. To maximize your email marketing results, maximize your email open rate. If recipients don’t open your emails, they can’t take the actions you want.  

Related: Six Tips for Maximizing Email Marketing Campaigns

Hack #1: Draft subject lines that are compelling and/or intriguing. Look through commercial emails you have opened recently. Often they have compelling words in their subject lines such as “free” or “sale.” Sometimes they provoke curiosity, with phrases such as “bad news.” Often they promise useful information you desire with titles starting with “how to”. Study headlines on the covers of your favorite magazines. Every headline is written to draw you in. Use those headlines as templates for your email subject lines.

2. Email click-through rates. The purpose for getting the recipient to open your email is the next action. Most often, you want the recipient to click on a link to buy your product or service, schedule a time to speak, download a report or whatever moves the process forward. Measuring the click through rate is crucial to improving your email marketing.

Hack #2: To maximize click through, offer multiple links promoting the same thing. The more opportunities to click, the more likely they will click. Make your links “pretty.” More people will click on “Download Your Free Report” than will click on http://www.your site.com/report/h5the.html

3. Earnings per click. Your recipient opened your email. You persuaded them to click. How much revenue or profit did you generate each time someone clicked? The next key metric to measure and improve is Earnings Per Click (EPC).

EPC is calculated by multiplying 1) the number of people who purchased and 2) the average purchase price, and then dividing this by the number of people who clicked.

Hack #3: To increase your EPC, offer numerous products and services to your email list over time. In doing so, you’ll determine which offers generate the highest EPC. Then simply promote these offer more often.

Related: 3 Scary Email Marketing Mistakes to Avoid

4. Email unsubscribe rates. It takes the same time to email 1,000 prospective customers as 10,000 prospective customers. When a recipient unsubscribes from your emails, you lose a potential customer you worked hard to get.

Hack #4: Your subject line is a promise. If it says “how to lose 10 pounds,” the content of your email must answer or support it. If it doesn’t, readers will be upset and unsubscribe.Offer readers value, not just a pitch. Don’t send emails just to promote your products, services and company. Use your expertise to offer good “how to” advice and educational content. The goal is for people to WANT to receive your emails. That occurs when you give them value.

Measuring results is crucial to improving results. Increasing performance in each of these four areas will quickly grow your email marketing revenues.

For example, consider Company A who starts with a list of 5,000 subscribers, has an open rate of 15 percent, a click through rate of 10 percent, an earnings per click of $25 and a monthly unsubscribe rate of 5 percent. In their second month of email marketing, this company would generate $1,781 in revenue.

Conversely, Company B, improves each of these rates by 20 percent. Also starting with 5,000 subscribers, they would have an open rate of 18 percent, click through rate of 12 percent, earnings per click of $30 and a monthly unsubscribe rate of 4 percent. In their second month of email marketing, this company would generate $3,110 in revenue. That’s 75 percent more than Company A.

Email marketing is a highly effective marketing strategy. Follow these tips to measure and maximize each of the four key metrics above and you quickly realize significant profits from your efforts.

Related: The 8-Second Challenge: Email Marketing for Our Shrinking Attention Span