diff --git a/src/atomicmin.rs b/src/atomicmin.rs
index 69a379f2..2df7028c 100644
--- a/src/atomicmin.rs
+++ b/src/atomicmin.rs
@@ -23,7 +23,7 @@ impl AtomicMin {
     }
 
     /// Unset value is usize_max
-    pub fn as_atomic_usize(&self) -> &AtomicUsize {
+    pub const fn as_atomic_usize(&self) -> &AtomicUsize {
         &self.val
     }
 
diff --git a/src/colors.rs b/src/colors.rs
index d81b1346..238bf262 100644
--- a/src/colors.rs
+++ b/src/colors.rs
@@ -45,7 +45,7 @@ impl Display for ColorType {
 impl ColorType {
     /// Get the code used by the PNG specification to denote this color type
     #[inline]
-    pub fn png_header_code(&self) -> u8 {
+    pub const fn png_header_code(&self) -> u8 {
         match self {
             ColorType::Grayscale { .. } => 0,
             ColorType::RGB { .. } => 2,
@@ -56,7 +56,7 @@ impl ColorType {
     }
 
     #[inline]
-    pub(crate) fn channels_per_pixel(&self) -> u8 {
+    pub(crate) const fn channels_per_pixel(&self) -> u8 {
         match self {
             ColorType::Grayscale { .. } | ColorType::Indexed { .. } => 1,
             ColorType::GrayscaleAlpha => 2,
@@ -66,12 +66,12 @@ impl ColorType {
     }
 
     #[inline]
-    pub(crate) fn is_rgb(&self) -> bool {
+    pub(crate) const fn is_rgb(&self) -> bool {
         matches!(self, ColorType::RGB { .. } | ColorType::RGBA)
     }
 
     #[inline]
-    pub(crate) fn is_gray(&self) -> bool {
+    pub(crate) const fn is_gray(&self) -> bool {
         matches!(
             self,
             ColorType::Grayscale { .. } | ColorType::GrayscaleAlpha
@@ -79,12 +79,12 @@ impl ColorType {
     }
 
     #[inline]
-    pub(crate) fn has_alpha(&self) -> bool {
+    pub(crate) const fn has_alpha(&self) -> bool {
         matches!(self, ColorType::GrayscaleAlpha | ColorType::RGBA)
     }
 
     #[inline]
-    pub(crate) fn has_trns(&self) -> bool {
+    pub(crate) const fn has_trns(&self) -> bool {
         match self {
             ColorType::Grayscale { transparent_shade } => transparent_shade.is_some(),
             ColorType::RGB { transparent_color } => transparent_color.is_some(),
diff --git a/src/headers.rs b/src/headers.rs
index 06671437..779b185a 100644
--- a/src/headers.rs
+++ b/src/headers.rs
@@ -27,7 +27,7 @@ impl IhdrData {
     /// Bits per pixel
     #[must_use]
     #[inline]
-    pub fn bpp(&self) -> usize {
+    pub const fn bpp(&self) -> usize {
         self.bit_depth as usize * self.color_type.channels_per_pixel() as usize
     }
 
@@ -38,7 +38,7 @@ impl IhdrData {
         let h = self.height as usize;
         let bpp = self.bpp();
 
-        fn bitmap_size(bpp: usize, w: usize, h: usize) -> usize {
+        const fn bitmap_size(bpp: usize, w: usize, h: usize) -> usize {
             ((w * bpp + 7) / 8) * h
         }
 
diff --git a/src/lib.rs b/src/lib.rs
index 28e5a4fb..c941fd66 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -249,7 +249,7 @@ impl Options {
         self
     }
 
-    fn apply_preset_2(self) -> Self {
+    const fn apply_preset_2(self) -> Self {
         self
     }
 
@@ -904,7 +904,7 @@ fn postprocess_chunks<T>(png: &mut PngData, opts: &Options, orig_ihdr: &IhdrData
 }
 
 /// Check if an image was already optimized prior to oxipng's operations
-fn is_fully_optimized(original_size: usize, optimized_size: usize, opts: &Options) -> bool {
+const fn is_fully_optimized(original_size: usize, optimized_size: usize, opts: &Options) -> bool {
     original_size <= optimized_size && !opts.force
 }
 
diff --git a/src/png/mod.rs b/src/png/mod.rs
index 49f7a89d..56c8b83f 100644
--- a/src/png/mod.rs
+++ b/src/png/mod.rs
@@ -241,13 +241,13 @@ impl PngImage {
 
     /// Return the number of channels in the image, based on color type
     #[inline]
-    pub fn channels_per_pixel(&self) -> usize {
+    pub const fn channels_per_pixel(&self) -> usize {
         self.ihdr.color_type.channels_per_pixel() as usize
     }
 
     /// Return the number of bytes per channel in the image
     #[inline]
-    pub fn bytes_per_channel(&self) -> usize {
+    pub const fn bytes_per_channel(&self) -> usize {
         match self.ihdr.bit_depth {
             BitDepth::Sixteen => 2,
             // Depths lower than 8 will round up to 1 byte
@@ -472,7 +472,7 @@ fn write_png_block(key: &[u8], chunk: &[u8], output: &mut Vec<u8>) {
 }
 
 // Integer approximation for i * log2(i) - much faster than float calculations
-fn ilog2i(i: u32) -> u32 {
+const fn ilog2i(i: u32) -> u32 {
     let log = 32 - i.leading_zeros() - 1;
     i * log + ((i - (1 << log)) << 1)
 }