delivered the opinion of the Court.
At the time petitioner committed the crime for which he was convicted, Florida’s sentencing guidelines would have resulted in a presumptive sentence of 314 to 414 years’ imprisonment. At the time petitioner was sentenced, the revised guidelines called for a presumptive sentence of 514 to 7 years in prison. The trial court applied the guidelines in effect at the time of sentencing and imposed a 7-year sentence. The question presented is whether application of these amended *425 guidelines in petitioner’s case is unconstitutional by virtue of the Ex Post Facto Clause.
t — I
In 1983, the Florida Legislature enacted legislation replacing Florida’s system of indeterminate sentencing with a sentencing guidelines scheme intended “to eliminate unwarranted variation in the sentencing process. ” Fla. Rule Crim. Proc. 3.701(b) (1983). See 1983 Fla. Laws, ch. 83-216. Under the sentencing statute, a guidelines commission was responsible for “the initial development of a statewide system of sentencing guidelines.” Fla. Stat. §921.001(1) (1983). Once the commission had made its recommendation, the Supreme Court of Florida was to develop a final system of guidelines. These guidelines were to become effective for crimes committed on or after October 1, 1983. Fla. Stat. §921.001(4)(a) (1983).
The sentencing statute authorized the guidelines commission to “meet annually or at the call of the chairman to review sentencing practices and recommend modifications to the guidelines.” Fla. Stat. §921.001(3) (1983). Before the convening of the legislature each year, the commission was to make its recommendations regarding the need for changes in the guidelines. The Supreme Court of Florida then could revise the sentencing guidelines to conform to all or part of the commission’s recommendations. The sentencing law provided, however, that such revisions would become effective “only upon the subsequent adoption by the Legislature of legislation implementing the guidelines as then revised.” Fla. Stat. §921.001(4)(b) (1983).
In accordance with this legislation, the Supreme Court of Florida developed sentencing guidelines that went into effect on October 1, 1983. See
In re Rules of Criminal Procedure (Sentencing Guidelines),
The presumptive sentence range was “assumed to be appropriate for the composite score of the offender.” Fla. Rule Crim. Proc. 3.701(d)(8) (1983). Within the recommended range, the sentencing judge had discretion to fix the sentence “without the requirement of a written explanation.” Ibid. If the sentencing judge wished to depart from the guideline range, however, the judge had to give clear and convincing reasons in writing for doing so:
“Departures from the presumptive sentence should be avoided unless there are clear and convincing reasons to warrant aggravating or mitigating the sentence. Any sentence outside of the guidelines must be accompanied by a written statement delineating the reasons for the departure. Reasons for deviating from the guidelines shall not include factors relating to either instant offense or prior arrests for which convictions have not been obtained.” Fla. Rule Crim. Proc. 3.701(d)(ll) (1983).
The “clear and convincing” standard was construed as requiring reasons “of such weight as to produce in the mind of the judge a firm belief or conviction, without hesitancy, that departure is warranted.”
State
v.
Mischler,
Petitioner was convicted in August 1984 on counts of sexual battery with slight force, a second-degree felony, Fla.
*427
Stat. §794.011(5) (Supp. 1984); burglary with an assault, a felony of the “first degree punishable by . . . life,” Fla. Stat. §810.02 (1983); and petit theft, a misdemeanor, Fla. Stat. §812.014(2)(c) (1983). On April 25, 1984, when these offenses were committed, the sentencing guidelines adopted October 1,1983, were still in effect. On May 8,1984, however, the Supreme Court of Florida proposed several revisions to the sentencing guidelines. See
Florida Bar: Amendment to Rules of Criminal Procedure (§.701, S.988
— Sentencing
Guidelines),
Only two changes made in the revised guidelines are relevant here. First, the guidelines changed the definition of “primary offense” from the offense wdth “the highest statutory degree,” to the offense which results in “the most severe sentence range.” See
*428 At petitioner’s sentencing hearing on October 2, 1984, the State contended that the revised guidelines should apply in determining petitioner’s sentence. Alternatively, the State argued that if the sentencing judge applied the earlier guidelines, he should depart from the guidelines’ range and impose a 7-year sentence. Id., at 8-9. The sentencing judge, rejecting petitioner’s ex post facto argument, ruled that the revised guidelines should apply. Concluding that he would “stay within the new guidelines,” the judge imposed a 7-year term of imprisonment for the sexual assault count. Id., at 10. Petitioner received a concurrent 7-year sentence on the burglary count, and time served on the misdemeanor charge. Id., at 6, 11.
On appeal, the Florida District Court of Appeal, relying on this Court’s decision in
Weaver
v.
Graham,
The Supreme Court of Florida reversed.
*429
We granted certiorari,
II
Article I of the United States Constitution provides that neither Congress nor any State shall pass any “ex post facto Law.” See Art. I, §9, cl. 3; Art. I, §10, cl. 1. Our understanding of what is meant by
ex post facto
largely derives from the case of
Calder
v.
Bull,
“1st. Every law that makes an action done before the passing of the law, and which was innocent when done, criminal; and punishes such action. 2d. Every law that aggravates a crime, or makes it greater than it was, when committed. 3d. Every law that changes the punishment, and inflicts a greater punishment, than the law annexed to the crime, when committed. 4th. Every law that alters the legal rules of evidence, and receives less, or different testimony, than the law required at the time of the commission of the offense, in order to convict the offender.” Id., at 390 (emphasis omitted).
Accord,
Dobbert
v.
Florida, supra,
at 292, quoting
Beazell
v.
Ohio,
Justice Chase explained that the reason the
Ex Post Facto
Clauses were included in the Constitution was to assure that federal and state legislatures were restrained from enacting arbitrary or vindictive legislation. See
Our test for determining whether a criminal law is ex post facto derives from these principles. As was stated in Weaver, to fall within the ex post facto prohibition, two critical elements must be present: first, the law “must be retrospective, that is, it must apply to events occurring before its enactment”; and second, “it must disadvantage the offender affected by it.” Id., at 29. We have also held in Dobbert v. Florida, supra, that no ex post facto violation occurs if a change does not alter “substantial personal rights,” but merely changes “modes of procedure which do not affect matters of substance.” Id., at 293. See Beazell v. Ohio, supra, at 170-171. Respondent contends that the revised sentencing law is neither impermissibly retrospective, nor to petitioner’s disadvantage; respondent also contends that the revised sentencing law is merely a procedural change. We consider these claims in turn.
A law is retrospective if it “changes the legal consequences of acts completed before its effective date.” Weaver, supra, at 31. Application of the revised guidelines law in petitioner’s case clearly satisfies this standard. Respondent nevertheless contends that the ex post facto concern for retrospective laws is not violated here because Florida’s sen *431 tencing statute “on its face provides for continuous review and recommendation of changes to the guidelines. ” Brief for Respondent 27-28. Relying on our decision in Dobbert, respondent argues that it is sufficient that petitioner was given “fair warning” that he would be sentenced pursuant to the guidelines then in effect on his sentencing date. Brief for Respondent 28.
In our view,
Dobbert
provides scant support for such a pinched construction of the
ex post facto
prohibition. In
Dobbert,
the capital sentencing statute in effect at the time the murders took place later was held to be invalid. In rejecting the defendant’s argument that imposition of the death penalty therefore was a change in punishment from the punishment “in effect” when the crimes were committed, the Court concluded that
ex post facto
concerns were satisfied because the statute on the books at the time Dobbert committed the crimes warned him of the specific punishment Florida prescribed for first-degree murders. See
It is “axiomatic that for a law to be
ex post facto
it must be more onerous than the prior law.”
Dobbert, supra,
at 294. Looking only at the change in primary offense points, the revised guidelines law clearly disadvantages petitioner and similarly situated defendants. See
Respondent maintains that the change in guidelines laws is not disadvantageous because petitioner “cannot show definitively that he would have gotten a lesser sentence.” Tr. of Oral Arg. 29. This argument, however, is foreclosed by our decision in
Lindsey
v.
Washington,
Petitioner plainly has been “substantially disadvantaged” by the change in sentencing laws. To impose a 7-year sentence under the old guidelines, the sentencing judge would have to depart from the presumptive sentence range of 314 to 414 years. As a result, the sentencing judge would have to provide clear and convincing reasons in writing for the departure, on facts proved beyond a reasonable doubt, and his determination would be reviewable on appeal. By contrast, because a 7-year sentence is within the presumptive range *433 under the revised law, the trial judge did not have to provide any reasons, convincing or otherwise, for imposing the sentence, and his decision was unreviewable. Thus, even if the revised guidelines law did not “technically . . . increase . . . the punishment annexed to [petitioner’s] crime,” Lindsey, supra, at 401, it foreclosed his ability to challenge the imposition of a sentence longer than his presumptive sentence under the old law. Petitioner therefore was “substantially disadvantaged” by the retrospective application of the revised guidelines to his crime.
Finally, even if a law operates to the defendant’s detriment, the
ex post facto
prohibition does not restrict “legislative control of remedies and modes of procedure which do not affect matters of substance.”
Dobbert,
Although tííe distinction between substance and procedure might sometimes prove elusive, here the change at issue appears to have little about it that could be deemed procedural. The 20% increase in points for sexual offenses in no wise alters the method to be followed in determining the appropriate sentence; it simply inserts a larger number into the same equation. The comments of the Florida Supreme Court acknowledge that the sole reason for the increase was to punish sex offenders more heavily: the amendment was intended to,
*434
and did, increase the “quantum of punishment” for category 2 crimes. See
Respondent objects that it is misleading to view the change in the revised guidelines apart from the sentencing scheme as a whole. Relying largely on decisions by the Courts of Appeals sustaining the United States Parole Commission’s guidelines against
ex post facto
claims, respondent urges that the revised guidelines “merely guide and channel” the sentencing judge’s discretion. Brief for Respondent 35. See,
e. g., Wallace
v.
Christensen,
We find the federal parole guidelines cases inapposite. The courts that have upheld the retrospective application of federal parole guidelines have articulated several reasons why the
ex post facto
prohibition does not apply. The majority of these courts have held that the federal parole guidelines are not “laws” for purposes of the
Ex Post Facto
Clause. See,
e. g., Wallace
v.
Christensen, supra,
at 1553-1554 (citing cases). Other courts have found that the guidelines merely rationalize the exercise of statutory discretion, and that retrospective application of the guidelines thus does not violate the
Ex Post Facto
Clause. See,
e. g., Warren
v.
United States Parole Comm’n, supra,
at 149,
None of the reasons given in the federal parole cases even arguably applies here. First, the revised sentencing law is a law enacted by the Florida Legislature, and it has the force and effect of law. Cf.
Williams
v.
State,
r — H I — I I — I
The law at issue in this case, like the law in Weaver, “makes more onerous the punishment for crimes committed before its enactment.” Weaver, supra, at 36. Accordingly, we find that Florida’s revised guidelines law, 1984 Fla. Laws, ch. 84-328, is void as applied to petitioner, whose *436 crime occurred before the law’s effective date. We reverse the judgment of the Supreme Court of Florida, and remand the case for further proceedings not inconsistent with this opinion.
It is so ordered.
