If a gin and tonic is your go-to drink, with a few tweaks you can enjoy this cocktail while still sticking to your diet plan. A standard 4 oz gin and tonic composed of 1 oz gin and 3 oz tonic comes in around 100 calories. If you are looking to make this drink even healthier, switch out the tonic for a diet tonic. When crafted with diet tonic, a 4 oz glass of this drink has roughly 35 calories.