Pet Dehydration Deficit Calculator
When an animal is dehydrated, the fluid deficit is replaced on top of ongoing maintenance needs. This calculator takes the body weight, the veterinarian-assessed percentage dehydration, and a daily maintenance rate, and returns the estimated deficit and the total replacement volume for the day. The percentage dehydration and maintenance rate are clinical inputs you supply: the tool performs arithmetic only and is not a substitute for a veterinary fluid plan.
Deficit formula
Deficit (mL) = weight (kg) * dehydration percent * 10
Maintenance (mL) = weight (kg) * maintenance rate (mL/kg/day)
Total first-day volume = deficit + maintenance
The factor 10 follows because 1 percent of body weight is about 10 mL per kilogram of fluid. Ongoing abnormal losses, if any, would be added separately by the veterinarian.
Worked example
A 10 kg dog assessed at 8 percent dehydration with 60 mL/kg/day maintenance: deficit = 10 * 8 * 10 = 800.00 mL. Maintenance = 10 * 60 = 600.00 mL. Total first-day volume = 800 + 600 = 1,400.00 mL.
Dehydration deficit: frequently asked questions
How is a fluid deficit estimated?
The deficit in millilitres equals body weight in kilograms times the percentage dehydration times 10. This comes from the rule that each 1 percent of body weight lost as fluid is about 10 mL per kilogram. For example, a 10 kg animal that is 8 percent dehydrated has a deficit of about 800 mL.
How is the percentage dehydration found?
Percentage dehydration is assessed clinically by a veterinarian from signs such as skin tenting, mucous membrane moisture, eye position and pulse quality. It is not measured by this tool: you enter the assessed percentage. Mild dehydration is around 5 percent and severe is 10 to 12 percent or more.
Does this replace a veterinary fluid plan?
No. Rehydration is given over a period set by the veterinarian, alongside maintenance fluids and any ongoing losses, and depends on the patient's cardiac and renal status. This calculator only estimates the deficit and adds a maintenance figure you supply.
Official sources
- U.S. Food and Drug Administration: Animal and Veterinary.
- The deficit relationship follows from the definition that a percentage of body weight lost as water equals that fraction of mass in fluid volume.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.