Veterinary Dehydration Calculator (Fluid Deficit & Replacement Rate)
Estimate the fluid deficit (% body weight) from clinical signs and calculate the mL volume to replace - either over 6 hours or 24 hours, on top of maintenance fluid rate.
Patient weight (kg)
Worst clinical sign present
How to use this tool
Mapping signs to percent dehydration
Percent dehydration is estimated, not measured. Use the worst clinical sign present: tacky mucous membranes alone ≈ 5%; mild skin tent + tacky MM ≈ 6%; sunken eyes + skin tent + dry MM ≈ 8%; tachycardia, weak pulses, prolonged CRT ≈ 10%; collapse / decompensated shock ≈ 12%.
The math
Fluid deficit (L) = body weight (kg) × % dehydration / 100. A 20 kg dog at 8% dehydrated has a 1.6 L deficit (1600 mL). Add this to maintenance (~50-60 mL/kg/day) and ongoing losses (vomit, diarrhoea) for the full fluid plan.
6-hour vs 24-hour replacement
Faster replacement (6 hours) for haemodynamically stable patients with no cardiac or renal compromise. Slower (24 hours) for patients with chronic kidney disease, cardiac disease, or hyponatraemia that requires gradual correction.
Shock vs dehydration
Decompensated shock requires immediate volume resuscitation, not deficit calculation. Give 1/4 of the calculated shock dose (dog 60-90 mL/kg, cat 40-60 mL/kg) IV over 15 minutes, reassess, repeat as needed - then re-estimate dehydration.
Frequently asked questions
How do you assess dehydration in dogs and cats?
Estimate from clinical signs: tacky mucous membranes ≈ 5%, mild skin tent ≈ 6%, sunken eyes + skin tent ≈ 8%, weak pulses and prolonged CRT ≈ 10%, collapse ≈ 12%. Percent dehydration is estimated, not measured - use the worst sign present.
How is fluid deficit calculated?
Deficit (L) = body weight (kg) × % dehydration / 100. A 20 kg dog at 8% dehydrated has a 1.6 L deficit. Add this to maintenance fluid rate (~50-60 mL/kg/day) and any ongoing losses.
Should I replace the deficit in 6 or 24 hours?
6 hours for haemodynamically stable patients with no cardiac or renal compromise. 24 hours for patients with chronic kidney disease, cardiac disease, or hyponatraemia that requires gradual correction.