Blood Transfusion Volume Calculator for Dogs and Cats
Calculate the volume of whole blood or packed red cells needed to raise a patient's PCV to a target level, using the standard weight-based transfusion formula for dogs and cats.
Species
Body weight (kg)
Current PCV (%)
Desired PCV (%)
Donor blood PCV (%)
Default of 40% is a rough placeholder - enter the actual PCV of your donor unit if known.
How to use this tool
The formula
Volume needed (mL) = Body weight (kg) × [90 for dogs, 60 for cats] × (Desired PCV − Current PCV) ÷ Donor PCV.
The constant (90 for dogs, 60 for cats) reflects each species' approximate blood volume per kilogram of body weight, built into the standard transfusion formula. Enter the donor unit's actual PCV when known - whole blood is often around 35-45%, while packed red cells run considerably higher.
Before you transfuse
Confirm blood type compatibility and crossmatch as appropriate before administering any blood product. Cats should always be typed and crossmatched given the risk of a fatal first-transfusion reaction. Recheck PCV after the transfusion to confirm the expected rise.
Frequently asked questions
How is transfusion volume calculated?
Volume needed (mL) = body weight (kg) × [90 for dogs, 60 for cats] × (desired PCV − current PCV) ÷ donor PCV. For example, a 20 kg dog with a current PCV of 15% and a desired PCV of 25%, given blood with a donor PCV of 40%: Volume = 20 × 90 × (25 − 15) ÷ 40 = 450 mL.
Why is the constant different for dogs (90) versus cats (60)?
It reflects each species' approximate blood volume per kg body weight combined with the standard transfusion formula's derivation - dogs have a higher blood volume per kg than cats, which is baked into the differing constants.
Do I need to crossmatch before every transfusion?
Cats should always be typed and crossmatched given the risk of a fatal first-transfusion reaction from naturally occurring alloantibodies. Dogs should be crossmatched if previously transfused or of unknown transfusion history - see this site's blood type reference for the full explanation.
How fast should a transfusion be given?
Start slow - roughly 0.25 mL/kg/hr for the first 15-30 minutes to watch for a reaction - then increase toward the target rate, generally keeping total rate at or under about 4-6 mL/kg/hr in a normovolemic patient unless treating hypovolemic shock.