diff --git a/crates/fayalite/src/int.rs b/crates/fayalite/src/int.rs
index d6f4cfa..4f41e53 100644
--- a/crates/fayalite/src/int.rs
+++ b/crates/fayalite/src/int.rs
@@ -18,7 +18,10 @@ use std::{
     fmt,
     hash::Hash,
     marker::PhantomData,
-    ops::{Add, BitAnd, BitOr, BitXor, Bound, Mul, Neg, Not, Range, RangeBounds, Shl, Shr, Sub},
+    ops::{
+        Add, BitAnd, BitOr, BitXor, Bound, Mul, Neg, Not, Range, RangeBounds, RangeInclusive, Shl,
+        Shr, Sub,
+    },
 };
 
 #[derive(Clone, Eq, PartialEq, Hash, Default)]
@@ -1003,6 +1006,66 @@ impl DynUIntType {
     pub fn mask(self) -> BigUint {
         self.modulo() - 1u8
     }
+    /// gets the smallest `UInt` that fits `v` losslessly
+    pub fn for_value(v: impl Into<BigUint>) -> Self {
+        let v: BigUint = v.into();
+        Self::new(v.bits().try_into().expect("too big"))
+    }
+    /// gets the smallest `UInt` that fits `r` losslessly, panics if `r` is empty
+    #[track_caller]
+    pub fn range(r: Range<impl Into<BigUint>>) -> Self {
+        let start: BigUint = r.start.into();
+        let end: BigUint = r.end.into();
+        assert!(!end.is_zero(), "empty range");
+        Self::range_inclusive(start..=(end - 1u8))
+    }
+    /// gets the smallest `UInt` that fits `r` losslessly, panics if `r` is empty
+    #[track_caller]
+    pub fn range_inclusive(r: RangeInclusive<impl Into<BigUint>>) -> Self {
+        let (start, end) = r.into_inner();
+        let start: BigUint = start.into();
+        let end: BigUint = end.into();
+        assert!(start <= end, "empty range");
+        // no need to check `start`` since it's no larger than `end`
+        // so must not take more bits than `end`
+        Self::for_value(end)
+    }
+}
+
+impl DynSIntType {
+    /// gets the smallest `SInt` that fits `v` losslessly
+    pub fn for_value(v: impl Into<BigInt>) -> Self {
+        let v: BigInt = v.into();
+        Self::new(
+            match v.sign() {
+                Sign::Minus => {
+                    // account for sign bit and for the minimum value of an `SInt`
+                    // being the negative of the maximum value minus one.
+                    v.not().bits().checked_add(1).expect("too big")
+                }
+                Sign::NoSign => 0,
+                Sign::Plus => v.bits(),
+            }
+            .try_into()
+            .expect("too big"),
+        )
+    }
+    /// gets the smallest `SInt` that fits `r` losslessly, panics if `r` is empty
+    #[track_caller]
+    pub fn range(r: Range<impl Into<BigInt>>) -> Self {
+        let start: BigInt = r.start.into();
+        let end: BigInt = r.end.into();
+        Self::range_inclusive(start..=(end - 1))
+    }
+    /// gets the smallest `SInt` that fits `r` losslessly, panics if `r` is empty
+    #[track_caller]
+    pub fn range_inclusive(r: RangeInclusive<impl Into<BigInt>>) -> Self {
+        let (start, end) = r.into_inner();
+        let start: BigInt = start.into();
+        let end: BigInt = end.into();
+        assert!(start <= end, "empty range");
+        Self::new(Self::for_value(start).width.max(Self::for_value(end).width))
+    }
 }
 
 impl<Signed: GenericConstBool> sealed::Sealed for DynIntType<Signed> {}