diff --git a/README.markdown b/README.markdown
index a7b6b13cd..9041b8823 100644
--- a/README.markdown
+++ b/README.markdown
@@ -136,9 +136,7 @@ The author is Alessandro Ranellucci.
         
       Accuracy options:
         --layer-height      Layer height in mm (default: 0.4)
-        --first-layer-height-ratio
-                            Multiplication factor for the height to slice and print the first
-                            layer with (> 0, default: 1)
+        --first-layer-height Layer height for first layer (mm or %, default: 100%)
         --infill-every-layers
                             Infill every N layers (default: 1)
       
diff --git a/lib/Slic3r.pm b/lib/Slic3r.pm
index 4cb84906c..efe5fd3ae 100644
--- a/lib/Slic3r.pm
+++ b/lib/Slic3r.pm
@@ -95,7 +95,8 @@ our $scaling_factor         = 0.000001;
 our $resolution             = 0.01;
 our $small_perimeter_length = (6.5 / $scaling_factor)*2*PI;
 our $layer_height           = 0.4;
-our $first_layer_height_ratio = 1;
+our $first_layer_height     = '100%';  # mm or %
+our $_first_layer_height    = undef;   # mm (computed)
 our $infill_every_layers    = 1;
 
 # flow options
diff --git a/lib/Slic3r/Config.pm b/lib/Slic3r/Config.pm
index 8b51f7dea..de3f9fce6 100644
--- a/lib/Slic3r/Config.pm
+++ b/lib/Slic3r/Config.pm
@@ -186,9 +186,9 @@ our $Options = {
         cli     => 'layer-height=f',
         type    => 'f',
     },
-    'first_layer_height_ratio' => {
-        label   => 'First layer height ratio',
-        cli     => 'first-layer-height-ratio=f',
+    'first_layer_height' => {
+        label   => 'First layer height (mm or %)',
+        cli     => 'first-layer-height=f',
         type    => 'f',
     },
     'infill_every_layers' => {
@@ -520,7 +520,7 @@ sub load {
         
         # handle legacy options
         next if $ignore{$key};
-        if ($key =~ /^(?:extrusion_width|bottom_layer_speed)_ratio$/) {
+        if ($key =~ /^(?:extrusion_width|bottom_layer_speed|first_layer_height)_ratio$/) {
             $key = $1;
             $key =~ s/^bottom_layer_speed$/first_layer_speed/;
             $val = $val =~ /^\d+(\.\d+)?$/ ? ($val*100) . "%" : 0;
@@ -571,9 +571,12 @@ sub validate {
     die "--layer-height must be a multiple of print resolution\n"
         if $Slic3r::layer_height / $Slic3r::scaling_factor % 1 != 0;
     
-    # --first-layer-height-ratio
-    die "Invalid value for --first-layer-height-ratio\n"
-        if $Slic3r::first_layer_height_ratio < 0;
+    # --first-layer-height
+    die "Invalid value for --first-layer-height\n"
+        if $Slic3r::first_layer_height !~ /^(?:\d+(?:\.\d+)?)%?$/;
+    $Slic3r::_first_layer_height = $Slic3r::first_layer_height =~ /^(\d+(?:\.\d+)?)%$/
+        ? ($Slic3r::layer_height * $1/100)
+        : $Slic3r::first_layer_height;
     
     # --filament-diameter
     die "Invalid value for --filament-diameter\n"
@@ -585,9 +588,7 @@ sub validate {
     die "--layer-height can't be greater than --nozzle-diameter\n"
         if $Slic3r::layer_height > $Slic3r::nozzle_diameter;
     die "First layer height can't be greater than --nozzle-diameter\n"
-        if ($Slic3r::layer_height * $Slic3r::first_layer_height_ratio) > $Slic3r::nozzle_diameter;
-    die "First layer height can't be zero or negative\n"
-        if ($Slic3r::layer_height * $Slic3r::first_layer_height_ratio) <= 0;
+        if $Slic3r::_first_layer_height > $Slic3r::nozzle_diameter;
     
     if ($Slic3r::extrusion_width) {
         $Slic3r::flow_width = $Slic3r::extrusion_width =~ /^(\d+(?:\.\d+)?)%$/
diff --git a/lib/Slic3r/GUI/SkeinPanel.pm b/lib/Slic3r/GUI/SkeinPanel.pm
index 6ea6f6dd6..b61c40804 100644
--- a/lib/Slic3r/GUI/SkeinPanel.pm
+++ b/lib/Slic3r/GUI/SkeinPanel.pm
@@ -40,7 +40,7 @@ sub new {
         },
         accuracy => {
             title => 'Accuracy',
-            options => [qw(layer_height first_layer_height_ratio infill_every_layers)],
+            options => [qw(layer_height first_layer_height infill_every_layers)],
         },
         print => {
             title => 'Print settings',
diff --git a/lib/Slic3r/Layer.pm b/lib/Slic3r/Layer.pm
index e886e49a6..a85a68c17 100644
--- a/lib/Slic3r/Layer.pm
+++ b/lib/Slic3r/Layer.pm
@@ -16,6 +16,10 @@ has 'id' => (
 
 has 'slicing_errors' => (is => 'rw');
 
+has 'slice_z' => (is => 'lazy');
+has 'print_z' => (is => 'lazy');
+has 'height'  => (is => 'lazy');
+
 # collection of spare segments generated by slicing the original geometry;
 # these need to be merged in continuos (closed) polylines
 has 'lines' => (
@@ -74,28 +78,25 @@ has 'fills' => (
 );
 
 # Z used for slicing
-sub slice_z {
+sub _build_slice_z {
     my $self = shift;
+    
     if ($self->id == 0) {
-        return ($Slic3r::layer_height * $Slic3r::first_layer_height_ratio) / 2 / $Slic3r::scaling_factor;
+        return $Slic3r::_first_layer_height / 2 / $Slic3r::scaling_factor;
     }
-    return (($Slic3r::layer_height * $Slic3r::first_layer_height_ratio)
-        + (($self->id-1) * $Slic3r::layer_height)
-        + ($Slic3r::layer_height/2)) / $Slic3r::scaling_factor;
+    return ($Slic3r::_first_layer_height + (($self->id-1) * $Slic3r::layer_height) + ($Slic3r::layer_height/2))
+        / $Slic3r::scaling_factor;  #/
 }
 
 # Z used for printing
-sub print_z {
+sub _build_print_z {
     my $self = shift;
-    return (($Slic3r::layer_height * $Slic3r::first_layer_height_ratio)
-        + ($self->id * $Slic3r::layer_height)) / $Slic3r::scaling_factor;
+    return ($Slic3r::_first_layer_height + ($self->id * $Slic3r::layer_height)) / $Slic3r::scaling_factor;
 }
 
-sub height {
+sub _build_height {
     my $self = shift;
-    return $self->id == 0
-        ? ($Slic3r::layer_height * $Slic3r::first_layer_height_ratio)
-        : $Slic3r::layer_height;
+    return $self->id == 0 ? $Slic3r::_first_layer_height : $Slic3r::layer_height;
 }
 
 sub add_line {
diff --git a/lib/Slic3r/TriangleMesh.pm b/lib/Slic3r/TriangleMesh.pm
index 795c4ec4a..5def6e342 100644
--- a/lib/Slic3r/TriangleMesh.pm
+++ b/lib/Slic3r/TriangleMesh.pm
@@ -390,10 +390,9 @@ sub slice_facet {
     }
     
     # calculate the layer extents
-    my $first_layer_height = $Slic3r::layer_height * $Slic3r::first_layer_height_ratio;
-    my $min_layer = int((unscale($min_z) - ($first_layer_height + $Slic3r::layer_height / 2)) / $Slic3r::layer_height) - 2;
+    my $min_layer = int((unscale($min_z) - ($Slic3r::_first_layer_height + $Slic3r::layer_height / 2)) / $Slic3r::layer_height) - 2;
     $min_layer = 0 if $min_layer < 0;
-    my $max_layer = int((unscale($max_z) - ($first_layer_height + $Slic3r::layer_height / 2)) / $Slic3r::layer_height) + 2;
+    my $max_layer = int((unscale($max_z) - ($Slic3r::_first_layer_height + $Slic3r::layer_height / 2)) / $Slic3r::layer_height) + 2;
     Slic3r::debugf "layers: min = %s, max = %s\n", $min_layer, $max_layer;
     
     my $lines = {};  # layer_id => [ lines ]
diff --git a/slic3r.pl b/slic3r.pl
index 8dcd9a983..607b4ba20 100755
--- a/slic3r.pl
+++ b/slic3r.pl
@@ -180,9 +180,7 @@ $j
     
   Accuracy options:
     --layer-height      Layer height in mm (default: $Slic3r::layer_height)
-    --first-layer-height-ratio
-                        Multiplication factor for the height to slice and print the first
-                        layer with (> 0, default: $Slic3r::first_layer_height_ratio)
+    --first-layer-height Layer height for first layer (mm or %, default: $Slic3r::first_layer_height)
     --infill-every-layers
                         Infill every N layers (default: $Slic3r::infill_every_layers)