diff --git a/lib/Geo/Hash.pm b/lib/Geo/Hash.pm
index 12a2fe0..58ecbb8 100644
--- a/lib/Geo/Hash.pm
+++ b/lib/Geo/Hash.pm
@@ -2,6 +2,7 @@ package Geo::Hash;
 
 use warnings;
 use strict;
+use POSIX qw/ceil/;
 use Carp;
 
 =head1 NAME
@@ -16,6 +17,10 @@ This document describes Geo::Hash version 0.03
 
 our $VERSION = '0.03';
 
+use constant LOG2_OF_10  => log(10)  / log(2);
+use constant LOG2_OF_180 => log(180) / log(2);
+use constant LOG2_OF_360 => log(360) / log(2);
+
 =head1 SYNOPSIS
 
     use Geo::Hash;
@@ -58,14 +63,25 @@ sub _mid {
     return ( $ar->[$wh][0] + $ar->[$wh][1] ) / 2;
 }
 
-# The number of bits necessary to represent the specified number of
-# decimal digits
-sub _d2b { int( shift() * 3.32192809488736 + 1 ) }
-
-sub _bits_for_number {
+sub _num_of_decimal_places($) {
     my $n = shift;
     return 0 unless $n =~ s/.*\.//;
-    return _d2b( length $n );
+    return length $n;
+}
+
+sub _length_for_bits($$) {
+    my ( $bits, $is_lat ) = @_;
+    my $q = int( $bits / 5 );
+    my $r = $bits % 5;
+    if ( $r == 0 ) {
+        return $q * 2;
+    }
+    elsif ( $r <= ( $is_lat ? 2 : 3 ) ) {
+        return $q * 2 + 1;
+    }
+    else {
+        return $q * 2 + 2;
+    }
 }
 
 =head2 C<< precision >>
@@ -79,9 +95,11 @@ lat, lon pair.
 
 sub precision {
     my ( $self, $lat, $lon ) = @_;
-    my $lab = _bits_for_number( $lat ) + 8;
-    my $lob = _bits_for_number( $lon ) + 9;
-    return int( ( ( $lab > $lob ? $lab : $lob ) + 1 ) / 2.5 );
+    my $lat_bit = ceil( _num_of_decimal_places( $lat ) * LOG2_OF_10 + LOG2_OF_180 );
+    my $lon_bit = ceil( _num_of_decimal_places( $lon ) * LOG2_OF_10 + LOG2_OF_360 );
+    my $lat_len = _length_for_bits( $lat_bit, 1 );
+    my $lot_len = _length_for_bits( $lon_bit, 0 );
+    return $lat_len > $lot_len ? $lat_len : $lot_len;
 }
 
 =head2 C<< encode >>
diff --git a/t/precision.t b/t/precision.t
new file mode 100644
index 0000000..15cac8c
--- /dev/null
+++ b/t/precision.t
@@ -0,0 +1,17 @@
+use strict;
+use warnings;
+use Test::More;
+use Geo::Hash;
+
+my $gh = Geo::Hash->new;
+is $gh->precision('45', '-120'), 4;
+is $gh->precision('45.0', '-120'), 5;
+is $gh->precision('45', '-120.0'), 5;
+is $gh->precision('45.00', '-120'), 6;
+is $gh->precision('45', '-120.00'), 7;
+is $gh->precision('45.000', '-120'), 8;
+is $gh->precision('45', '-120.000'), 8;
+is $gh->precision('45.00000', '-120'), 10;
+is $gh->precision('45', '-120.00000'), 11;
+
+done_testing;