diff --git a/base/ryu/utils.jl b/base/ryu/utils.jl
index b9b4bac675cb45..656b28a4021788 100644
--- a/base/ryu/utils.jl
+++ b/base/ryu/utils.jl
@@ -372,23 +372,21 @@ end
 
 const POW10_OFFSET_2, MIN_BLOCK_2, POW10_SPLIT_2 = generateinversetables()
 
-bitlength(this) = Base.GMP.MPZ.sizeinbase(this, 2)
-
 @inline function pow5invsplit(::Type{Float64}, i)
     pow = big(5)^i
-    inv = div(big(1) << (bitlength(pow) - 1 + pow5_inv_bitcount(Float64)), pow) + 1
+    inv = div(big(1) << (ndigits(pow, base=2) - 1 + pow5_inv_bitcount(Float64)), pow) + 1
     return (UInt64(inv & ((big(1) << 64) - 1)), UInt64(inv >> 64))
 end
 
 @inline function pow5invsplit(::Type{Float32}, i)
     pow = big(5)^i
-    inv = div(big(1) << (bitlength(pow) - 1 + pow5_inv_bitcount(Float32)), pow) + 1
+    inv = div(big(1) << (ndigits(pow, base=2) - 1 + pow5_inv_bitcount(Float32)), pow) + 1
     return UInt64(inv)
 end
 
 @inline function pow5invsplit(::Type{Float16}, i)
     pow = big(5)^i
-    inv = div(big(1) << (bitlength(pow) - 1 + pow5_inv_bitcount(Float16)), pow) + 1
+    inv = div(big(1) << (ndigits(pow, base=2) - 1 + pow5_inv_bitcount(Float16)), pow) + 1
     return UInt32(inv)
 end