diff --git a/GNUmakefile.in b/GNUmakefile.in
index 7e3b25e..0a90819 100644
--- a/GNUmakefile.in
+++ b/GNUmakefile.in
@@ -31,8 +31,8 @@ MAIN_OBJ= \
 	$(ED25519_OBJ) \
 	keccak.c.o
 
-UTIL_CALCDIFF_OBJ= \
-	calcdiff.c.o
+UTIL_CALCEST_OBJ= \
+	calcest.c.o
 
 TEST_BASE64_OBJ= \
 	test_base64.c.o \
@@ -69,10 +69,11 @@ ALL_C= $(patsubst %.c.o,%.c,$(filter %.c.o %.c,$(ALL_O)))
 CLEAN_O= $(filter %.o,$(ALL_O))
 
 MAIN_LIB= -lpthread -lsodium @MAINLIB@
+UTIL_CALCEST_LIB= -lm
 TEST_ED25519_LIB= -lsodium
 
 MAIN_TGT= mkp224o
-UTIL_TGT= calcdiff
+UTIL_TGT= calcest
 TEST_TGT= test_base64 test_base32 test_base16 test_ed25519
 
 MAIN_EXE= $(patsubst %,%@EXEEXT@,$(MAIN_TGT))
@@ -90,8 +91,8 @@ all: $(ALL_EXE)
 mkp224o@EXEEXT@: $(MAIN_OBJ)
 	$(CC) $(LDFLAGS) $(CFLAGS) -o $@.tmp $^ $(MAIN_LIB) && $(MV) $@.tmp $@
 
-calcdiff@EXEEXT@: $(UTIL_CALCDIFF_OBJ)
-	$(CC) $(LDFLAGS) $(CFLAGS) -o $@.tmp $^ && $(MV) $@.tmp $@
+calcest@EXEEXT@: $(UTIL_CALCEST_OBJ)
+	$(CC) $(LDFLAGS) $(CFLAGS) -o $@.tmp $^ $(UTIL_CALCEST_LIB) && $(MV) $@.tmp $@
 
 test_base64@EXEEXT@: $(TEST_BASE64_OBJ)
 	$(CC) $(LDFLAGS) $(CFLAGS) -o $@.tmp $^ && $(MV) $@.tmp $@
diff --git a/calcest.c b/calcest.c
new file mode 100644
index 0000000..23e88de
--- /dev/null
+++ b/calcest.c
@@ -0,0 +1,43 @@
+#include <stdio.h>
+#include <math.h>
+
+/*
+ * as per scribblemaniac's explanation:
+ * t - number of trials
+ * n - character count
+ * p - probability
+ * condition: >=1 matches
+ * formula: t = log(1-p)/log(1-1/32^n)
+ * comes from:
+ *  distribution X~Binomial(t, 1/32^n)
+ *  P(X>=1)=p
+ */
+
+const double probs[] = { 0.5, 0.8, 0.9, 0.95, 0.99 };
+const int charcounts[] = { 2, 3, 4, 5, 6, 7 };
+
+int main(void)
+{
+	printf("   |");
+	for (int i = 0; i < sizeof(probs)/sizeof(probs[0]); ++i) {
+		printf(" %11d%% |",(int)((probs[i]*100)+0.5));
+	}
+	printf("\n");
+
+	printf("---+");
+	for (int i = 0; i < sizeof(probs)/sizeof(probs[0]); ++i) {
+		printf("--------------+");
+	}
+	printf("\n");
+
+	for (int i = 0; i < sizeof(charcounts)/sizeof(charcounts[0]); ++i) {
+		printf("%2d |",charcounts[i]);
+		for (int j = 0; j < sizeof(probs)/sizeof(probs[0]); ++j) {
+			double t = log2(1 - probs[j]) / log2(1 - (1 / pow(32,charcounts[i])));
+			printf(" %12.0f |",t);
+		}
+		printf("\n");
+	}
+
+	return 0;
+}