diff --git a/src/liballoc/boxed.rs b/src/liballoc/boxed.rs
index 13d4a0a1f0a08..e2c1af8a2dafe 100644
--- a/src/liballoc/boxed.rs
+++ b/src/liballoc/boxed.rs
@@ -17,7 +17,7 @@ use core::default::Default;
 use core::fmt;
 use core::intrinsics;
 use core::mem;
-use core::option::Option;
+use core::option::{Option, Some};
 use core::raw::TraitObject;
 use core::result::{Ok, Err, Result};
 
@@ -130,12 +130,22 @@ impl<T: fmt::Show> fmt::Show for Box<T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         (**self).fmt(f)
     }
+
+    #[inline]
+    fn formatter_len_hint(&self) -> Option<uint> {
+        (**self).formatter_len_hint()
+    }
 }
 
 impl fmt::Show for Box<Any+'static> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         f.pad("Box<Any>")
     }
+
+    #[inline]
+    fn formatter_len_hint(&self) -> Option<uint> {
+        Some(8)
+    }
 }
 
 #[cfg(test)]
diff --git a/src/libcollections/string.rs b/src/libcollections/string.rs
index 6843996a9e145..1d6a10f6a4c63 100644
--- a/src/libcollections/string.rs
+++ b/src/libcollections/string.rs
@@ -899,6 +899,11 @@ impl fmt::Show for String {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         self.as_slice().fmt(f)
     }
+
+    #[inline]
+    fn formatter_len_hint(&self) -> Option<uint> {
+        self.as_slice().formatter_len_hint()
+    }
 }
 
 #[experimental = "waiting on Hash stabilization"]
diff --git a/src/libcore/fmt/mod.rs b/src/libcore/fmt/mod.rs
index 7bab59960b0fe..9016aa860f203 100644
--- a/src/libcore/fmt/mod.rs
+++ b/src/libcore/fmt/mod.rs
@@ -14,10 +14,13 @@
 
 use any;
 use cell::{Cell, Ref, RefMut};
+use cmp;
 use collections::Collection;
+use f32;
 use iter::{Iterator, range};
 use kinds::Copy;
 use mem;
+use num::FPNormal;
 use option::{Option, Some, None};
 use ops::Deref;
 use result::{Ok, Err};
@@ -171,6 +174,11 @@ impl<'a> Show for Arguments<'a> {
 pub trait Show {
     /// Formats the value using the given formatter.
     fn fmt(&self, &mut Formatter) -> Result;
+
+    /// Returns a conservative estimate of the size of the formatted string.
+    ///
+    /// A return of `None` means a close estimate is costly or not feasible.
+    fn formatter_len_hint(&self) -> Option<uint> { None }
 }
 
 /// Format trait for the `b` character
@@ -260,7 +268,7 @@ macro_rules! uniform_fn_call_workaround {
             pub fn $name<T: $trait_>(x: &T, fmt: &mut Formatter) -> Result {
                 x.fmt(fmt)
             }
-            )*
+        )*
     }
 }
 uniform_fn_call_workaround! {
@@ -591,6 +599,7 @@ impl<'a, T: Show> Show for &'a mut T {
 }
 impl<'a> Show for &'a Show+'a {
     fn fmt(&self, f: &mut Formatter) -> Result { (*self).fmt(f) }
+    fn formatter_len_hint(&self) -> Option<uint> { #![inline] (*self).formatter_len_hint() }
 }
 
 impl Bool for bool {
@@ -705,20 +714,69 @@ macro_rules! floating(($ty:ident) => {
 floating!(f32)
 floating!(f64)
 
+fn formatter_len_hint_string(this: &&str) -> Option<uint> {
+    Some(this.len())
+}
+
+fn formatter_len_hint_bool(_: &bool) -> Option<uint> {
+    Some(5)
+}
+
+fn formatter_len_hint_char(_: &char) -> Option<uint> {
+    Some(4)
+}
+
+static LOG10_2: f32 = f32::consts::LOG10_E / f32::consts::LOG2_E;
+static SHIFTED_LOG10_2: i32 = (LOG10_2 * (1u << 16) as f32) as i32;
+
+fn formatter_len_hint_float_f32(this: &f32) -> Option<uint> {
+    use num::{Float, Signed};
+
+    match this.classify() {
+        FPNormal => {
+            let (_, exponent, _) = this.integer_decode();
+            // A fast approximate log_10. Add a small value at the end for the
+            // sign and decimal separator.
+            let log_10 = ((exponent as i32 + 23) * SHIFTED_LOG10_2) >> 16;
+            Some(cmp::max(log_10, -5).abs() as uint + 4)
+        }
+        // Otherwise, "+inf" is the longest string we might print.
+        _ => Some(4)
+    }
+}
+
+fn formatter_len_hint_float_f64(this: &f64) -> Option<uint> {
+    use num::{Float, Signed};
+
+    match this.classify() {
+        FPNormal => {
+            let (_, exponent, _) = this.integer_decode();
+            let log_10 = ((exponent as i32 + 52) * SHIFTED_LOG10_2) >> 16;
+            Some(cmp::max(log_10, -5).abs() as uint + 4)
+        }
+        // Otherwise, "+inf" is the longest string we might print.
+        _ => Some(4)
+    }
+}
+
 // Implementation of Show for various core types
 
-macro_rules! delegate(($ty:ty to $other:ident) => {
+macro_rules! delegate(($ty:ty to $other:ident $($suffix:ident)*) => {
     impl<'a> Show for $ty {
         fn fmt(&self, f: &mut Formatter) -> Result {
             (concat_idents!(secret_, $other)(self, f))
         }
+        #[inline]
+        fn formatter_len_hint(&self) -> Option<uint> {
+            (concat_idents!(formatter_len_hint_, $other, $($suffix),*)(self))
+        }
     }
 })
 delegate!(&'a str to string)
 delegate!(bool to bool)
 delegate!(char to char)
-delegate!(f32 to float)
-delegate!(f64 to float)
+delegate!(f32 to float _f32)
+delegate!(f64 to float _f64)
 
 impl<T> Show for *const T {
     fn fmt(&self, f: &mut Formatter) -> Result { secret_pointer(self, f) }
@@ -792,6 +850,11 @@ impl Show for () {
     fn fmt(&self, f: &mut Formatter) -> Result {
         f.pad("()")
     }
+
+    #[inline]
+    fn formatter_len_hint(&self) -> Option<uint> {
+        Some(2)
+    }
 }
 
 impl<T: Copy + Show> Show for Cell<T> {
@@ -804,12 +867,22 @@ impl<'b, T: Show> Show for Ref<'b, T> {
     fn fmt(&self, f: &mut Formatter) -> Result {
         (**self).fmt(f)
     }
+
+    #[inline]
+    fn formatter_len_hint(&self) -> Option<uint> {
+        (**self).formatter_len_hint()
+    }
 }
 
 impl<'b, T: Show> Show for RefMut<'b, T> {
     fn fmt(&self, f: &mut Formatter) -> Result {
         (*(self.deref())).fmt(f)
     }
+
+    #[inline]
+    fn formatter_len_hint(&self) -> Option<uint> {
+        (*(self.deref())).formatter_len_hint()
+    }
 }
 
 // If you expected tests to be here, look instead at the run-pass/ifmt.rs test,
diff --git a/src/libcore/fmt/num.rs b/src/libcore/fmt/num.rs
index eb8d684fe50fd..3b990540475dd 100644
--- a/src/libcore/fmt/num.rs
+++ b/src/libcore/fmt/num.rs
@@ -14,10 +14,13 @@
 
 #![allow(unsigned_negate)]
 
+use clone::Clone;
 use collections::Collection;
 use fmt;
 use iter::DoubleEndedIterator;
-use num::{Int, cast, zero};
+use mem::size_of;
+use num::{Int, Signed, cast, zero};
+use option::{Option, Some};
 use slice::{ImmutableSlice, MutableSlice};
 
 /// A type that represents a specific radix
@@ -146,13 +149,54 @@ pub fn radix<T>(x: T, base: u8) -> RadixFmt<T, Radix> {
     RadixFmt(x, Radix::new(base))
 }
 
+macro_rules! int_base_hint {
+    ($Trait:ident for $T:ident as $U:ident -> $Radix:ident; $log2log2base:expr, $abs:ident) => {
+        impl fmt::$Trait for $T {
+            fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+                $Radix.fmt_int(*self as $U, f)
+            }
+
+            fn formatter_len_hint(&self) -> Option<uint> {
+                let num = self.$abs();
+                let width = size_of::<$T>() * 8;
+                // Approximate log_2 of the target base.
+                let log2base = 1 << $log2log2base;
+
+                // Get the number of digits in the target base.
+                let binary_digits = width - (num | log2base as $T).leading_zeros();
+                Some(binary_digits / log2base)
+            }
+        }
+    };
+    // Use `clone` on uints as a noop method in place of abs.
+    ($Trait:ident for $T:ident as $U:ident -> $Radix:ident; $log2log2base:expr) => {
+        int_base_hint!($Trait for $T as $U -> $Radix; $log2log2base, clone)
+    }
+}
 macro_rules! radix_fmt {
-    ($T:ty as $U:ty, $fmt:ident) => {
+    ($T:ty as $U:ty, $fmt:ident, $abs:ident) => {
         impl fmt::Show for RadixFmt<$T, Radix> {
             fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-                match *self { RadixFmt(ref x, radix) => radix.$fmt(*x as $U, f) }
+                let &RadixFmt(x, radix) = self;
+                radix.$fmt(x as $U, f)
+            }
+
+            fn formatter_len_hint(&self) -> Option<uint> {
+                let &RadixFmt(num, radix) = self;
+                let num = num.$abs();
+                let width = size_of::<$T>() * 8;
+                // Approximate log_2 of the target base.
+                let log2base = 7 - radix.base().leading_zeros();
+
+                // Get the number of digits in the target base.
+                let binary_digits = width - (num | log2base as $T).leading_zeros();
+                Some(binary_digits / log2base + 1)
             }
         }
+    };
+    // Use `clone` on uints as a noop method in place of abs.
+    ($T:ty as $U:ty, $fmt:ident) => {
+        radix_fmt!($T as $U, $fmt, clone)
     }
 }
 macro_rules! int_base {
@@ -166,20 +210,20 @@ macro_rules! int_base {
 }
 macro_rules! integer {
     ($Int:ident, $Uint:ident) => {
-        int_base!(Show     for $Int as $Int   -> Decimal)
         int_base!(Signed   for $Int as $Int   -> Decimal)
         int_base!(Binary   for $Int as $Uint  -> Binary)
         int_base!(Octal    for $Int as $Uint  -> Octal)
         int_base!(LowerHex for $Int as $Uint  -> LowerHex)
         int_base!(UpperHex for $Int as $Uint  -> UpperHex)
-        radix_fmt!($Int as $Int, fmt_int)
+        int_base_hint!(Show for $Int as $Int -> Decimal; 1, abs)
+        radix_fmt!($Int as $Int, fmt_int, abs)
 
-        int_base!(Show     for $Uint as $Uint -> Decimal)
         int_base!(Unsigned for $Uint as $Uint -> Decimal)
         int_base!(Binary   for $Uint as $Uint -> Binary)
         int_base!(Octal    for $Uint as $Uint -> Octal)
         int_base!(LowerHex for $Uint as $Uint -> LowerHex)
         int_base!(UpperHex for $Uint as $Uint -> UpperHex)
+        int_base_hint!(Show for $Uint as $Uint -> Decimal; 1)
         radix_fmt!($Uint as $Uint, fmt_int)
     }
 }
diff --git a/src/libcoretest/fmt/mod.rs b/src/libcoretest/fmt/mod.rs
index 3f2208e6c2dad..d960afe5cdb2e 100644
--- a/src/libcoretest/fmt/mod.rs
+++ b/src/libcoretest/fmt/mod.rs
@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use core::fmt::Show;
+
 mod num;
 
 #[test]
@@ -16,3 +18,35 @@ fn test_format_flags() {
     let p = "".as_ptr();
     assert_eq!(format!("{:p} {:x}", p, 16u), format!("{:p} 10", p));
 }
+
+#[test]
+fn test_len_hints_float() {
+    let mut f = 5.0f32;
+    for _ in range(0u, 30) {
+        let s = format!("{}", f);
+        let len = f.formatter_len_hint().unwrap();
+        assert!(len >= s.len());
+        assert!(len <= 128);
+        f /= 10.0;
+    }
+    let mut f = 5.0f32;
+    for _ in range(0u, 30) {
+        let s = format!("{}", f);
+        let len = f.formatter_len_hint().unwrap();
+        assert!(len >= s.len());
+        assert!(len <= 128);
+        f *= 10.0;
+    }
+}
+
+#[test]
+fn test_len_hints_u64() {
+    let mut f = 1u64;
+    for _ in range(0u, 20) {
+        let s = format!("{}", f);
+        let len = f.formatter_len_hint().unwrap();
+        assert!(len >= s.len());
+        assert!(len <= 128);
+        f *= 10;
+    }
+}
diff --git a/src/librustrt/c_str.rs b/src/librustrt/c_str.rs
index 04a4e96ecc40e..97044695c70fe 100644
--- a/src/librustrt/c_str.rs
+++ b/src/librustrt/c_str.rs
@@ -315,6 +315,11 @@ impl fmt::Show for CString {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         String::from_utf8_lossy(self.as_bytes_no_nul()).fmt(f)
     }
+
+    #[inline]
+    fn formatter_len_hint(&self) -> Option<uint> {
+        Some(self.len())
+    }
 }
 
 /// A generic trait for converting a value to a CString.
diff --git a/src/libstd/ascii.rs b/src/libstd/ascii.rs
index fe2b8a15c7375..5ed423ddc0f54 100644
--- a/src/libstd/ascii.rs
+++ b/src/libstd/ascii.rs
@@ -182,6 +182,10 @@ impl<'a> fmt::Show for Ascii {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         (self.chr as char).fmt(f)
     }
+
+    fn formatter_len_hint(&self) -> Option<uint> {
+        Some(1)
+    }
 }
 
 /// Trait for converting into an ascii type.
diff --git a/src/libstd/to_string.rs b/src/libstd/to_string.rs
index c19fd81b57056..3f3daa9757631 100644
--- a/src/libstd/to_string.rs
+++ b/src/libstd/to_string.rs
@@ -16,6 +16,8 @@ The `ToString` trait for converting to strings
 
 #![experimental]
 
+use io::Writer;
+use io;
 use fmt;
 use string::String;
 
@@ -33,7 +35,9 @@ pub trait IntoStr {
 
 impl<T: fmt::Show> ToString for T {
     fn to_string(&self) -> String {
-        format!("{}", *self)
+        let mut output = io::MemWriter::with_capacity(self.formatter_len_hint().unwrap_or(128));
+        let _ = write!(&mut output, "{}", *self);
+        String::from_utf8(output.unwrap()).unwrap()
     }
 }