diff --git a/src/calc.rs b/src/calc.rs
new file mode 100644
index 0000000..0d49217
--- /dev/null
+++ b/src/calc.rs
@@ -0,0 +1,68 @@
+use num_traits::{PrimInt, Unsigned};
+
+use crate::{Decimal, DecimalError};
+
+#[derive(Debug, Clone)]
+pub struct RPNCalc<const SS: usize = 2, const DS: usize = 5, E = u16> {
+    stack: [Decimal<DS, E>; SS],
+    cur: usize,
+}
+
+impl<const SS: usize, const DS: usize, E: PrimInt> Default for RPNCalc<SS, DS, E> {
+    fn default() -> Self {
+        RPNCalc {
+            stack: [Decimal::default(); SS],
+            cur: 0,
+        }
+    }
+}
+
+impl<const SS: usize, const DS: usize, E: PrimInt + Unsigned> RPNCalc<SS, DS, E> {
+    pub fn enter(&mut self, val: Decimal<DS, E>) {
+        self.stack[self.cur] = val;
+        if self.cur == SS - 1 {
+            self.cur = 0;
+        } else {
+            self.cur += 1;
+        }
+    }
+
+    pub fn pop(&mut self) -> Decimal<DS, E> {
+        self.cur = if self.cur == 0 { SS - 1 } else { self.cur - 1 };
+        let ret = self.stack[self.cur];
+        self.stack[self.cur] = Decimal::default();
+        ret
+    }
+
+    pub fn add(&mut self) -> Result<Decimal<DS, E>, DecimalError> {
+        let a = self.pop();
+        let b = self.pop();
+        let a = f64::try_from(a)?;
+        let b = f64::try_from(b)?;
+        let res = Decimal::try_from(a + b)?;
+        self.enter(res);
+        Ok(res)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn test_add() -> Result<(), DecimalError> {
+        let mut calc = RPNCalc::<3, 5>::default();
+        calc.enter(Decimal::try_from(1337.42)?);
+        assert_eq!(calc.add()?, Decimal::try_from(1337.42)?);
+        calc.enter(Decimal::try_from(1337.42)?);
+        assert_eq!(calc.add()?, Decimal::try_from(1337.42 * 2.0)?);
+        calc.enter(Decimal::try_from(2.0)?);
+        calc.enter(Decimal::try_from(3.0)?);
+        assert_eq!(calc.add()?, Decimal::try_from(5.0)?);
+        assert_eq!(calc.add()?, Decimal::try_from(1337.42 * 2.0 + 5.0)?);
+        assert_eq!(calc.add()?, Decimal::try_from(1337.42 * 2.0 + 5.0)?);
+        assert_eq!(calc.add()?, Decimal::try_from(1337.42 * 2.0 + 5.0)?);
+        assert_eq!(calc.add()?, Decimal::try_from(1337.42 * 2.0 + 5.0)?);
+        assert_eq!(calc.add()?, Decimal::try_from(1337.42 * 2.0 + 5.0)?);
+        Ok(())
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index c8262af..1e3e90c 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,5 +1,7 @@
 #![no_std]
 
+pub mod calc;
+
 use core::{error::Error, fmt::Display};
 #[cfg(not(test))]
 use num_traits::float::FloatCore;
@@ -21,7 +23,7 @@ impl Display for DecimalError {
     }
 }
 
-#[derive(Debug, Clone, Copy)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub struct Decimal<const S: usize = 5, E = u16> {
     pub minus: bool,
     pub significant: [u8; S],