diff --git a/ana/predictor.go b/ana/predictor.go
index 81180e3..3c81f99 100644
--- a/ana/predictor.go
+++ b/ana/predictor.go
@@ -2,6 +2,7 @@ package ana
 
 import (
 	"maps"
+	"math"
 	"regexp"
 	"time"
 
@@ -27,7 +28,8 @@ func NewInterestPredictor(namePattern string, currencyPattern string, apy float6
 				if !currencyMatcher.MatchString(string(currency)) {
 					continue
 				}
-				result[name][currency] *= 1.0 + apy/12.0
+				percentOfYearPassed := float64(delta) / float64(time.Hour*365)
+				result[name][currency] *= math.Pow(1.0+apy/12.0, percentOfYearPassed)
 			}
 		}
 		return result
diff --git a/ana/predictor_test.go b/ana/predictor_test.go
new file mode 100644
index 0000000..2769735
--- /dev/null
+++ b/ana/predictor_test.go
@@ -0,0 +1,43 @@
+package ana
+
+import (
+	"testing"
+	"time"
+
+	"gogs.inhome.blapointe.com/ana-ledger/ledger"
+)
+
+func TestNewInterestPredictor(t *testing.T) {
+	name := "x"
+	curr := ledger.USD
+	cases := map[string]struct {
+		apy   float64
+		given ledger.Balances
+		delta time.Duration
+		want  ledger.Balances
+	}{
+		"zero": {
+			apy:   0,
+			given: ledger.Balances{name: ledger.Balance{curr: 100}},
+			delta: time.Hour * 24 * 365,
+			want:  ledger.Balances{name: ledger.Balance{curr: 100}},
+		},
+		"100%": {
+			apy:   1,
+			given: ledger.Balances{name: ledger.Balance{curr: 100}},
+			delta: time.Hour * 24 * 365,
+			want:  ledger.Balances{name: ledger.Balance{curr: 200}},
+		},
+	}
+
+	for name, d := range cases {
+		c := d
+		t.Run(name, func(t *testing.T) {
+			predictor := NewInterestPredictor(name, string(curr), c.apy)
+			got := predictor(c.given, c.delta)
+			if got.Debug() != c.want.Debug() {
+				t.Errorf("want\n\t%+v, got\n\t%+v", c.want, got)
+			}
+		})
+	}
+}