How is the MER Calculator different from the main Calorie Calculator?

They use the same formula (RER × MER factor). The main Calorie Calculator on the home page is the all-in-one version designed for everyday use. The MER Calculator is a standalone tool for users who want to explore the factor table directly, compare scenarios, or use a vet-prescribed MER factor rather than the one the calculator selects automatically.

Why is the cat MER factor lower than the dog factor for the same life stage?

The factor reflects real physiological and lifestyle differences. Adult cats — especially indoor spayed/neutered cats — have lower maintenance energy requirements relative to their RER than adult dogs of similar body size. This is well established in veterinary nutrition references and is reflected in the WSAVA and AAHA feeding guidelines.

How accurate is the MER factor approach?

The factor-based approach gives a good population-average starting estimate — reliable enough to set a starting portion — but individual metabolism varies by 20–30% around the formula's output. Treat the MER result as a starting point, monitor your pet's weight over 2–4 weeks, and adjust the portion up or down if weight is trending in the wrong direction.

Should I use MER or RER for a pet on a weight-loss plan?

Weight-loss targets are typically calculated from RER (often 80% of RER at current weight) rather than from MER, under veterinary guidance. MER-based targets are for healthy maintenance. If your vet has prescribed a specific calorie restriction, use the RER Calculator to get the baseline and apply the restriction as directed.

Does the MER factor change if my dog gains or loses weight?

The factor itself (based on life stage and activity) stays the same, but the RER it multiplies changes with weight. So yes — recalculate whenever you have a fresh weight, because both the RER and therefore the MER will shift.

How the MER Calculator Works | PetCalculator

Maintenance Energy Requirement (MER) is what a healthy, normally active pet actually needs to eat each day — RER multiplied by a life-stage and activity factor. This guide explains each factor in the table and how to pick the right one.