Fluid Therapy - Rehydration Calculations

Product Quick Finder

Choose a category or subcategory

There are many reasons for the need to provide fluid therapy to an animal in the veterinary practice. 

A patient may require fluids to maintain its body fluid balance over the course of a day as it may not be able to eat or drink, or a patient may need to have fluid maintained on a daily basis while hospitalized or over a shorter period of time, e.g., during a surgical procedure or the patient may need to have fluid to replace that lost through dehydration. Haemorrhage, medication administration, electrolyte replacement or even to provide energy are also factors that will be considered when looking to rehydrate a patient.

Once the fluid type has been chosen, we need to then work out the amount of fluid that needs to be replaced. Estimations and calculations are based on information gathered through accurate history taking, clinical examination and laboratory tests. The most reliable way to establish the degree of dehydration is judged on using as many criteria as possible. 

Evaluation is generally crude, in that some processes are personally subjective; however, the patient must be re-evaluated at least daily, allowing for fluid rates to be altered as the condition changes for the better or worse.

Assessing hydration status 

There are various measurements and observations used to determine a patient’s hydration status which including checking capillary refill time (CRT), mucous membranes, eye position, dermal elasticity as well as checking packed cell volume (PCV) will all help to formulate a plan for rehydrating the patient [1,4,5]

CRT may be slower to return to normal in a dehydrated patient compared to the normal <2 seconds. To check the CRT the gum is pressed with a finger with the time taken for the gum colour to return to normal being recorded. Dry mucous membranes are indicative of a dehydrated patient and can also be assessed while carrying out CRT; gums should normally be moist to the touch [4,5].

Observing eye position within the socket – retraction of the eye back into its socket will present with a look ‘sunken’ and indicates dehydrated of the patient. Eyes should be bright, and turgidly sit within the socket [4,5,].

Dermal elasticity – Indicated by gently pinching the patient’s skin (called ‘tenting’). In a healthy patient, skin will resume its normal form almost immediately. In cases of dehydration there will be less elasticity within the skin and it will take longer to resume a normal position. This assessment is objective and depends on the age, condition and amount of subcutaneous fat the animal carries [4,5].

A PCV is usually part of the blood sampling process; whilst a PCV gives the volume of red blood cells in the blood, it also indicates the hydration status of the patient. The normal PCV for a cat or dog is 32–45%. A higher PCV (> 45%) would indicate dehydration, as the top layer of the plasma sample, the fluid component of the blood, is reduced so that the relative proportion of red blood cells is greater. When assessing dehydration, the rule is that a 1% increase in PCV is equivalent to a water loss of 10 ml/kg [4,5].


When we look to rehydrating a patient, we must consider three factors that will count towards our overall daily calculations for the patient:

• Maintenance – What they would normally require throughout a normal day

• Dehydration – How dehydrated the patient is and what % factor we need to rehydrate them by

• Ongoing losses – What they continue to lose due to illness

We need to factor all of these together to start to rehydrate our patients.


Fluid maintenance is about the same for all species. However, the proportion of body fluids varies between individuals. Animals with increased body fat tend to have a lower percentage of fluid, as fat displaces water within the cells. Alternatively, young animals have a higher percentage of fluid due to the solid elements of the body still being underdeveloped. Animals with heart conditions need to have reduced fluid rates to reduce potential for increased pressure placed on the heart by increased fluid volumes circulating the body. Lactating animals for example may have needs much greater than maintenance, these must all be considered.

Calculating maintenance:

Our first calculation aspect is based on the daily maintenance amount that an animal would normally use/lose and we must make sure we provide that as the base calculation. You will see suggested maintenance rates [2,3,4,5] to be estimated from 40-60ml/kg/day and should be adopted to suit the purpose. Clinics will usually have set rates to use or cheat sheets with the calculations already worked out but it is important to have an understanding of how fluid rates are calculated.

Fluid rates will change hourly or daily depending on the health of the animal. Monitoring of the animal by the vet nurse is very important as it is you who will generally note significant or even insignificant changes in the wellbeing of your patient and be able to feed that back to the Vet for reassessment.

Maintenance fluid loss in terms of body weight is calculated as a linear relationship of body weight and daily fluid requirements:

Insensible losses:

  • 20 ml/kg bodyweight/day – respiration and sweating
  • 10–20 ml/kg bodyweight/day – faeces


Sensible losses:

  • 20 ml/kg bodyweight/day – urine


The losses from the respiratory tract and the skin cannot be regulated and are described as insensible or inevitable water loss. Losses from the kidneys are linked to thirst and osmoregulatory mechanisms involved in the maintenance of extracellular fluid volume. This is considered sensible water loss and equate to the total loss of fluid from the body to be estimated ~ 40–60 ml/kg bodyweight/day or 2-3ml/kg/hr cats and 2-6ml/kg/hr for dogs [4,5].

The above consideration to maintenance calculations uses a linear relationship of body weight and daily fluid requirements but does not consider the surface area to volume ratios of an animals metabolic rate. An allometric scale formula does predict metabolic water requirements more accurately for smaller <10kg and larger >30kg patients (similar to nutrition calculation formulas) [1]. The calculation is represented by the following formula for dogs: 132 x bodyweight (kg) (0.75) and for cats: 80 x bodyweight (kg) (0.75) [4,5].


When a patient is vomiting, has a temperature or diarrhoea, they are losing more fluid and cannot preserve their maintenance fluids required to maintain haemostasis (fluid balance) within the body. Conditions that may cause abnormal hydration status and fluid redistribution include starvation, acute renal failure, diabetes insipidus, diabetes mellitus, toxic shock, pleural effusion, haemorrhage and septicaemia to name a few.

Clinical signs associated with dehydration:

Clinical examination – Dehydration 

Various texts [1,2,3,4,5] will show slight variations on the percentages and clinical signs; however, they are all along the same lines and reiterates the ‘subjective’ nature of the assessment.

<5 % 

May show slight increase in urine concentration or be slightly polydipsic, but is usually undetectable

5-6 %

Skin slightly doughy, with a slight reduction in dermal elasticity (tenting), eyes may have a slight sunken look, mucous membranes may have a ‘tacky’ consistency, CRT will be increased and may have a rapid weak pulse

6-9 %

Tented skin remains in place, sunken eyes, anuria, cold extremities, depression, weak pulse

10-12 %

Considerable loss of skin turgor, eyes sunken in orbits, prolonged refill time, extremely dry mucous membranes, tachycardia, weak thready pulses, altered mentation and hypotensive

Progression of shock into comatose state and death is usually inevitable [4,5,7]

It is important to note that not all animals will exhibit all of the mentioned signs.

Calculating dehydration is based on using the percentages above to express the fluid deficit and can be carried out in the following way:

For fluid deficit to be in millilitres (mL) the formula is % dehydration x bodyweight x 10 

e.g., 6% x 40kg x 10 = 2400ml

For fluid deficit to be in litres (L) the formula is 0.0_ dehydration x bodyweight

e.g., 0.06 x 40kg = 2.4lt

Calculating in mL is good for small animals and calculating in L is good for dealing with large animals as the numbers are easier to factor into the whole calculation.


Ongoing losses

Ongoing losses refer to vomiting, diarrhoea, polyuria, pyrexia etc. This is over and above covering the animal’s normal maintenance and replacing the fluid deficit, as they are losing more fluid, due to the disease process they are suffering. Remembering we do not include normal sensible urine loss as it is factored into the maintenance amount; however, we do account for polyuria as this is increased urination over and above normal. The following are estimates only and must always be checked with the vet prior to adding to the calculations.

  • Diarrhoea: 100 ml/kg/day [6]
  • Vomiting: 4ml/kg/episode of vomiting or 4ml/kg/day [6]
  • Pyrexia: 3ml/kg/1°degree rise above reference temp
  • PCV: 10ml per 1% rise above ref. range [7]
  • Fluids in general – if on a ‘vetbed’, or pad – weigh the pad, minus the initial starting weight will equate to every gram being equivalent to 1ml of fluid loss.

Animal’s must be reviewed every 4-6hours to determine any necessary adjustments. Let’s look at the following examples:

A 12 kg puppy has had diarrhoea estimated at 5 times daily for 2 days: 100ml/kg/day x 12kg = 1200ml/day

A 2 kg cat has vomited 3 times today: 4ml/kg x 3 episodes x 2kg = 24ml/day

A dog’s temperature is 2 degrees above normal it weighs 10kg: 3ml x 10kg x 2° increase = 60ml/day

The same dog’s PCV is increased by 2% 10ml x 2 =20ml/day

Another option for calculating ongoing losses is to apply 1mL/kg//hr [5] this negates the formality of using the above calculations; however, the practice should have protocols to guide you on their chosen specific calculation regimes.

Consideration should also be given to increasing fluid rates may have a negative effect especially in gastrointestinal and urinary production increases. If the fluid output increases due to fluid input then fluids should be decreased, and losses no longer replaced. This again will be the Vet’s decision.



Daily replacement calculation

Now we need to bring all three calculations together to get the animals total daily  replacement fluid amount of Maintenance + Fluid deficit + ongoing losses = total daily replacement

For example:

A dog weighing 20kg has been vomiting 5 times a day for 3 days.  It is showing clinical symptoms typical of 7% dehydration. Calculate the total fluid replacement required over the next 24 hrs if maintenance is 50ml/kg/day:

  • Maintenance x bodyweight

• 50ml/kg/day x 20kg = 1000ml required to maintain normal fluids over 24 hours

  • % dehydration x bodyweight x 10

• 20kg x 7 x 10 = 1400 ml total fluid deficit

  • BW x ongoing losses per 24 hrs (4ml/kg/episode)

• 20kg x 4ml = 80ml x 5 times in 24 hrs

• 80ml x 5 times in 24 hrs = 400ml total ongoing losses

  • Adding all together = 2800ml over 24 hrs


Let’s try one more:

A cat weighing 3 kg has not eaten for two days. It is showing clinical symptoms typical of 5% dehydration and has had one ‘bout’ of diarrhoea. Calculate the total fluid replacement required over the next 24 hrs if maintenance is 50ml/kg/day

  • Maintenance x bodyweight

•3kg x 50ml = 150 ml total maintenance required over 24 hrs 

  • BW x % dehydration x 10

•3kg x 5 x 10 = 150ml total fluid deficit 

  • BW x ongoing losses = 3kg x 100ml/kg = 300ml 
  • Adding all together = 600ml required over 24 hrs 

We need to factor all of these together to start to not only rehydrate our patients but maintain their normal maintenance levels and replace ongoing losses. Patients must be re-evaluated at least once daily as the fluid balance corrects or worsens; therefore, the calculated fluid requirements will change too.

It is ideal that the same person/s monitor the patient so that subtle changes may be noted or that thorough records are kept helping assist with reassessment.

Patients hydration status must be monitored closely while on fluids so they are not overhydrated.

If you would like to learn more about calculations and carry out practice calculations, you can go to The Animal Academy – Short courses - calculations series to enrol in the practice calculations courses they have availabl

1. Davis, H., Jensen, T., Johnson, A., Knowles, P., Meyer, R., Rucinsky, R., & Shafford, H. (2013). 2013 AAHA/AAFP fluid therapy guidelines for dogs and cats. Journal of the American animal hospital association, 49(3), 149-159. Accessed December 2022 from: https://www.aaha.org/globalassets/02-guidelines/fluid-therapy/fluid_therapy_guidelines.pdf 
3.Cooper, B. (2020). BSAVA Textbook of veterinary nursing. Wiley.
4.Thorp, A. (2018). Fluid Calculations: Keeping a Balance. Todays Veterinary Nurse. Accessed December 2022 from: https://todaysveterinarynurse.com/internal-medicine/fluid-calculations-keeping-a-balance/
5. Lyons, B. M., & Waddell, L. S. Fluid Therapy in Hospitalized Patients: Electrolyte Abnormalities and Fluid Balance.
6. Brown, A. J., & Otto, C. M. (2008). Fluid therapy in vomiting and diarrhea. Veterinary Clinics of North America: Small Animal Practice, 38(3), 653-675.
7. Aspinall, V., & Ackerman, N. (2016). Aspinall's complete textbook of veterinary nursing. 3rd edition / Amsterdam, Elsevier.