diff --git a/src/QEDbase.jl b/src/QEDbase.jl
index c247838..33e1503 100644
--- a/src/QEDbase.jl
+++ b/src/QEDbase.jl
@@ -62,6 +62,7 @@ export AbstractDefinitePolarization, AbstractIndefinitePolarization
 export PolarizationX, PolX, PolarizationY, PolY, AllPolarization, AllPol
 export AbstractDefiniteSpin, AbstractIndefiniteSpin
 export SpinUp, SpinDown, AllSpin
+export multiplicity
 
 using StaticArrays
 using LinearAlgebra
diff --git a/src/particles/particle_spin_pol.jl b/src/particles/particle_spin_pol.jl
index 27e8973..c08a060 100644
--- a/src/particles/particle_spin_pol.jl
+++ b/src/particles/particle_spin_pol.jl
@@ -177,3 +177,15 @@ y-polarized
 struct PolarizationY <: AbstractDefinitePolarization end
 const PolY = PolarizationY
 Base.show(io::IO, ::MIME"text/plain", ::PolY) = print(io, "y-polarized")
+
+"""
+    multiplicity(spin_or_pol)
+
+Return the number of spins or polarizations respresented by `spin_or_pol`, e.g. `multiplicity(SpinUp()) == 1`, but `multiplicity(AllSpin()) = 2`.
+
+"""
+function multiplicity end
+multiplicity(::AbstractDefinitePolarization) = 1
+multiplicity(::AbstractDefiniteSpin) = 1
+multiplicity(::AbstractIndefinitePolarization) = 2
+multiplicity(::AbstractIndefiniteSpin) = 2
diff --git a/test/particles.jl b/test/particles.jl
index 17a1559..6129c1b 100644
--- a/test/particles.jl
+++ b/test/particles.jl
@@ -52,6 +52,19 @@ test_broadcast(x::AbstractSpinOrPolarization) = x
     end
 end
 
+@testset "multiplicity of spins or pols" begin
+    @testset "single" begin
+        @testset "$spin_or_pol" for spin_or_pol in (SpinUp(), SpinDown(), PolX(), PolY())
+            @test multiplicity(spin_or_pol) == 1
+        end
+    end
+    @testset "multiple" begin
+        @testset "$spin_or_pol" for spin_or_pol in (AllSpin(), AllPol())
+            @test multiplicity(spin_or_pol) == 2
+        end
+    end
+end
+
 @testset "fermion likes" begin
     @testset "fermion" begin
         struct TestFermion <: Fermion end