Brain ratio
Divide Brain B by Brain A. In the supplied example, 1,320 / 1,200 produces a size ratio of 1.10.
1.10×
A bigger brain does not make every region equally bigger.
Compare proportional isometry with a user-chosen allometric exponent. The calculation is transparent; the biology still needs evidence.
Brain A1,200 mL
Brain B1,320 mL
Whole-brain change+10.00%
Interactive comparison
Change the exponent.
Watch the region respond.
The exponent is an assumption you control, not a measured fact supplied by this tool. Use a fitted value from a real study when drawing biological conclusions.
Computed region BSublinear scaling
107.41 mL
The region grows 7.41% while the whole brain grows 10.00%.
Brain A
→Brain B
Isometric region B110.00 mL
Difference from isometry−2.59 mL
Regional change+7.41%
100 × (1320 / 1200)^0.75 = 107.41Because the exponent is below 1, regional volume changes more slowly than whole-brain volume in this scenario.
Read the relationship
One ratio, raised to a power.
The power-law comparison separates the overall size change from the way one region is assumed to scale with it.
Isometry sets the exponent to one.
The region follows the whole-brain ratio exactly: 10% more brain implies 10% more region. It is the comparison line, not a universal biological rule.
b = 1.00
Below one
A positive exponent below one produces slower regional change than whole-brain change.
At one
An exponent of one preserves the same proportional change.
Above one
An exponent above one produces faster regional change in the direction of the brain-size ratio.
Interpret with care
The calculator shows arithmetic, not an empirical law.
An exponent becomes biologically meaningful only when it is estimated from appropriate measurements, species, developmental stages, methods, and uncertainty.
Scenario, not estimate
This tool never supplies a “correct” exponent. You provide the assumption so its mathematical consequences stay visible.
Scale is not cause
A scaling relationship can summarize how measurements covary. It does not by itself explain development, function, evolution, or mechanism.
Keep the evidence attached
Export the calculation, then record where the exponent came from, which population it describes, and how uncertainty changes the interpretation.
Take the assumptions with the answer.
The JSON brief preserves the exact inputs, formula, result, comparison, and caution so the calculation can be checked later.