Why is the weight loss calorie target set at 80% of RER, not MER?

Using 80% of RER (the base metabolic rate without any lifestyle multiplier) creates a meaningful calorie deficit for most pets without going so low that muscle loss and nutritional gaps become risks. This approach is the standard starting point in WSAVA weight-management protocols. It's deliberately conservative because going below RER significantly increases the chance of lean mass loss and nutritional shortfalls.

How accurate is the weeks-to-target estimate?

The estimate is a planning guide based on a consistent calorie deficit or surplus, which assumes accurate food measurement and no other changes. In practice, actual rate of loss or gain varies by individual metabolism, activity level, and how precisely food is measured. Monthly weigh-ins let you course-correct.

My cat needs to lose weight -- is it safe to do this without a vet?

Weight loss in cats carries a specific risk of hepatic lipidosis (fatty liver disease) if done too aggressively. We strongly recommend involving your vet before starting any calorie-restriction plan for a cat -- even a moderate one. Your vet can set a safe calorie target, monitor bloodwork, and adjust the plan based on how your cat responds.

Can I use this calculator for a dog that needs to gain weight?

Yes -- entering a target weight higher than the current weight switches the calculator to a gain plan using 110-120% of MER. However, unexplained weight loss in a dog that is eating normally warrants a vet visit to rule out illness before simply increasing food.

Should I recalculate midway through a weight-loss plan?

Yes -- as your pet loses weight, their RER decreases, so 80% of RER is a smaller number than it was at the start of the plan. Recalculating every 4-6 weeks, or whenever your pet has lost 5% of their starting weight, keeps the calorie target calibrated to their actual current weight.

How the Weight Loss & Gain Calculator Works | PetCalculator

The Weight Loss & Gain Calculator estimates how many weeks a safe, gradual weight change will take, using a 1-2% per week rate of change and adjusted calorie targets from veterinary weight-management protocols.