diff --git a/src/Downloads.jl b/src/Downloads.jl
index 6a6c5cb..fce01ae 100644
--- a/src/Downloads.jl
+++ b/src/Downloads.jl
@@ -391,7 +391,7 @@ end
 p_func(progress::Nothing, input::ArgRead, output::ArgWrite) = nothing
 
 arg_read_size(path::AbstractString) = filesize(path)
-arg_read_size(io::IOBuffer) = io.size - io.ptr + 1
+arg_read_size(io::Base.GenericIOBuffer) = bytesavailable(io)
 arg_read_size(::Base.DevNull) = 0
 arg_read_size(::Any) = nothing
 
diff --git a/test/runtests.jl b/test/runtests.jl
index 7142738..1f0323c 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -463,6 +463,14 @@ include("setup.jl")
         Downloads.download("https://httpbingo.org/drip"; downloader=dl)
         Downloads.download("https://httpbingo.org/drip"; downloader=dl)
     end
+
+    @testset "Input body size" begin
+        # Test mechanism to detect the body size from the request(; input) argument
+        @test Downloads.arg_read_size(@__FILE__) == filesize(@__FILE__)
+        @test Downloads.arg_read_size(IOBuffer("αa")) == 3
+        @test Downloads.arg_read_size(IOBuffer(codeunits("αa"))) == 3  # Issue #142
+        @test Downloads.arg_read_size(devnull) == 0
+    end
 end
 
 Downloads.DOWNLOADER[] = nothing